m- 


\  . 


CONVERSATIONS 

ON 
IN  WHICH 

THE  ELEMENTS  OF  THAT  SCIENCE 

ARE 

FAMILIARLY  EXPLAINED, 

AND  ADAPTED  TO  THE 

COMPREHENSION  OF  YOUNG  PUPILS. 

BY  THE  AUTHOR  OF  CONVERSATIONS  ON  CHEMISTRY,  AND 
CONVERSATIONS  ON  POLITICAL  ECONOMY. 


WITH 

ADDITIONAL  ILLUSTRATIONS 

AND 

APPROPRIATE   QUESTIONS 

FOR  THE 

EXAMINATION  OF  SCHOLARS, 

BT 

REV.  J.  L.  BLAKE,  A.  M. 

Principal  of  the  Literary  Seminary,  Concord,  New  Hampshire. 


FlfTB  AMERICAJr  KDITIOJr. 


HARTFORD : 

PRIKTBD  BT  GEORGB  «OODWIN. 

1823. 


DISTRICT  OF  NEW-HAMPSHIRE  :-TO  WIT. 

District  Clerk's  Office, 

»********«  T)  E  it  remembei-ed,  that  on  the  twenty-seventh  day  of  March,  A.  D, 

»    L    S     *  JlJ   18-23,  and  in  the  forty-seventh  year  of  the   Independence   of  the 

*       •     *    J  Umted  States  of  America,  JOHN  LAURIS  BLAKE, of  the  said  District, 

»«««*««*«*  ^3th  deposited  in  this  office  the  title  of  a  book,  the  right  whereof  he 

claims  as  author  in  the  words  following,  viz.  "  Conversations  on  Natural  Philosophy, 

*'  in  which  the  elements  of  that  science  are  familiarly  explained,  and  adapted  to  the 

*' comprehension  of  young  pupils.    Illustrated  with  plates,     ay  the  author  of  Conver- 

**  satious  on  Chemistry,  and  Conversations  on  Political  Economy.     With  additional 

*'  ilhisirationsand  appropriate  questions  for  the  examination  of  scholars,  by  Rev.  J.  L. 

•*  BLAKE,  A.  >L  Principal  of  the  Literary  Seminai7,  Concord,  New  Hampshire." 

In  conformity  to  the  act  of  the  Congress  of  the  United  States,  entitled  "  An  act  for 
the  encouragement  of  learning  by  securing  the  copies  of  maps,  charts  and  books  to  the 
authors  and  proprietors  of  such  copies,  during  the  times  therein  mentioned;"  and  also 
to  an  act  ir-ntitkd  '•  an  act  supplementary  to  an  act  entitled  an  act  for  the  encourage- 
ment of  1  .arning,  by  securing  the  copies  of  maps,  charts  and  books  to  the  authors  and 
proprietorsof  such  copies,  durinj?  the  times  therein  mentioned,  and  extending  the  ben- 
efits thereof  to  the  arts  of  designing,  engraving  and  etching  historical  and  other  prints." 
WILLIAM  CLAGGETT,  Clerk  of  the 

District  of  New- Hampshire. 
A  true  copy  of  record, 

Attest,  WILLIAM  CkAGGETT,  Clerk. 


PREFACE. 


■'••©^5©  *<*■" 


It  is  with  increased  diffidence  that  the  author  offers 
this  little  work  to  the  public.  The  encouraging  recep- 
tion which  the  Conversations  on  Chemistry,  and 
Pohtical  Economy  have  met  with,  has  induced  her 
to  venture  on  publishing  a  short  course  on  Natural 
Philosophy  ;  but  not  without  the  greatest  apprehen- 
sions for  its  success.  Her  ignorance  of  mathematics, 
and  the  imperfect  knov/ledge  of  natural  philosophy 
which  that  disadvantage  necessarily  implies,  renders 
her  fully  sensible  of  her  incompetency  to  treat  the 
subject  in  any  other  way  than  in  the  form  of  a  familiar 
explanation  of  the  first  elements,  for  the  use  of  very 
young  pupils.  It  is  the  hope  of  having  done  this  in  a 
manner  that  may  engage  their  attention,  which  encour- 
ages her  to  offer  them  these  additional  lessons. 

They  are  intended,  in  a  course  of  elementary  science, 
to  precede  the  Conversations  on  Chemistry  ;  and' 
were  actually  written  previous  to  either  of  her  former 
publications. 

[by  the  AMERICAN  EDITOR.} 

The  Conversations  on  Natural  Philosophy,  by  the 
author  of  Conversations  on  Chemistry,  are  probably 
better  adapted  to  the  minds  of  females  and  to  persons 
generally  who  are  only  to  acquire  the  great  principles 
of  natural  science  independent  of  abstruse  demonstra- 
tions, than  any  other  treatise  pubhshed  on  tlie  subject. 
A  persuasion  of  this,  induced  the  author  of  the  following 
Questions  for  the  examination  of  scholars  in  that  work, 
to  introduce  it  into  the  Seminary  of  which  he  has  the 
superintendence.  He  soon  found,  however,  that  his 
pupils  were  frequently  embarrassed  in  not  knowing,  to 
what  particular  parts  they  were  chiefly  to  direct  the 


iv  PREFACE. 

attention,  committing  to  memory  what  was  not  neces- 
sary and  omitting  what  was,  causing  thereby  a  great 
loss  of  time  as  well  as  of  improvement.  This  led  to 
the  present  system  of  questions,  by  the  use  of  which  it 
was  quickly  ascertained,  that  his  pupils  could  complete 
their  lessons  in  about  half  the  time  they  before  needed, 
and  far  better  than  they  could  before  possibly  get  them. 
And  this  determined  the  author  to  publish  and  append 
them  to  the  work  ;  and  in  coming  to  the  determination, 
it  occurred  to  him,  that  some  additional  illustrations 
might  advantageousl}^  be  introduced,  which,  together 
with  the  questions,  would  form  a  valuable  improvement. 

J.  L.  BLAKE. 
Concord,  N.  H.  1823. 


INDEX. 


A. 


Am,  15, 19,  32,  56,  145,  173,  192. 
Air-Pump,  36,  147. 
Angle,  49. 
— ,  acute,  50. 

,  obtuse,  50. 

of  incidence,  51,  170,  184. 

of  reflection,  51,  163,  169, 

184. 

of  vision,  178,  180. 

Aphelion,  82. 
Arctic  circle,  99,  1 07 
Atmosphere,  111,  138,  145,  155, 

173. 

,  reflection  of,  158. 

,  color  of,  203. 

— — — ,   refraction  of,    190, 

193. 
Attraction,  14, 19,  28,  190. 

■  ,  of  coiiesion,  19,  39, 126, 

145. 

— ,  of  gravitation,  23,  36, 

77,  87,  103,  122,  145. 
Avenue,  180, 
Auditory  Nerve,  164. 
Axis,  84. 

of  motion,  54,  60. 

of  the  earth,  99,  106. 

of  mirrors,  186. 

-  of  a  lens,  195. 

B. 

Balloon,  35. 
Barometer,  149. 
Bass,  164. 
Bladder,  147. 
Bodies,  14. 

'     ,  elastic,  45,  55. 

,  luminous,  166. 

— ,  sonorous,  160,  164. 

,  fall  of,  28,  31,  35,  42. 

,  opaque,  166, 190. 

-,  transparent,  167, 190. 


Camera  obscura,  173,  183,  212. 
Capillary  tubes,  22. 
Centre,  54. 

of  Gravity,  54,  57,  59,  61, 

122. 

»—  of  motion,  54,  60,  123. 

of  magnitude,  54,58. 

Centrifugal  force,  55,  79, 102, 122. 

Centripetal  force,  55,  79.- 

Ceres,  91. 

Circle,  48,  101,  103, 

Circular  motion,  53,  79. 

Clouds,  137. 

Colors,  28,  195. 

Comets,  92. 

Compression,  47. 

Concord,  165. 

Constellation,  92. 

Convergent  rays,  185,  187. 

Crystals,  16. 

Cylinder,  58. 


Bulk,  20. 


22 


Day,  84,  112. 

Degrees,  49,  101,  106,  181. 

of  latitude,  101,  119. 

of  longitude,  101,  119. 

Density,  20. 
Diagonal,  53. 
Diameter,  101. 
Diurnal,  85. 
Discords,  164. 
Divergent  rays,  185. 
Divisibility,  14,  16, 

E. 

Earth,  23,77,90,  90,98. 

Echo,  163. 

Eclipse,  117,  120,  168. 

Ecliptic,  93,  100. 

Elastic  bodies,  45, 46. 

fluids,  19,  33,  127,  145, 


;!j4 


INDEX. 


Ellipsis,  81. 

Essential  properties,  14, 
Exhalations,  17. 
Extension,  14,  15. 
Equator,  99. 
Equinox,  107,  108. 

,  precession  of,  114. 

Eye,  173. 


Fall  of  bodies,  28,  SI,  35,  42. 
Figure,  14,  16. 
Fluids,  126. 

,  elastic,  127,  145. 

,  equilibrium  of,  128, 150. 

,  pressure  of,  129, 140, 150. 

Flying,  45. 
Focus,  186. 

'  of  convex  mirrors,  187. 

— of  concave,  188. 

•   ■  of  a  lens,  194. 
Force,  38. 

,  centrifugal,  55,  79, 102,  122. 

,  centripetal,  55,  79. 

— ^  of  projection,  o6,  78. 

of  gravity,  23,  77,  87.  Ill, 

145. 
Fountains,  143. 
Friction,  74, 144. 
Frigid  zone,  100,  107. 
Fulcrum,  60. 

G. 

General  properties  of  bodies,  14. 
Georgium-Sidus,  91. 
Glass,  194. 

,  refraction  of,  194. 

— — ,  burning,  198. 

Gold,  13:3, 

Gravity,  23,  28,  36,  38,  42,  56,  57. 

II. 

ilarmonv,  156. 
Heat,  20,  110. 
Hemisphere,  99,  lCt7. 
llvdi'ometer,  136. 
Hydrostatics,  126. 

T. 

lini';;^eo.t  tin;  retina,  1743  182. 
Image  r-:? versed,  1  76. 
Hi  i.Iriin  mi'iTor.  183. 


'    in  convex  ditto,  185, 
— -  in  concave,  185. 
Impenetrability,  14. 
Inclined  plane,  60,72 
Inertia,  14, 18,  39. 


J. 


Juno,  91. 
Jupiter,  9§,  120. 


Lake,  142. 
Latitude,  101,  119. 
Lens,  194. 

' ,  convex,  194. 

,  concave,  195. 

Lever,  60. 

-,  first  order,  64. 

'-,  second  ditto,  66. 

,  third  ditto,  66. 

Light,  166. 
'     ■,  pencil  of,  167. 

,  reflected,  169. 

,  of  the  moon,  171. 

— -,  refraction  of,  190. 

,  absorption  of,  198. 

Liquids,  127. 
Longitude,  101,  119. 
Luminous  bodies,  166. 
Lunar  month,  116. 
-  eclipse,  117. 

M. 

Machine,  60,  72,  74. 
Magic  Lanthorn,  213. 
jNTars,  90. 
Matter,  l4,  44. 
Mechanics,  60. 
Mediums,  167,  190. 
]Melody,  165. 
Mercury  planet?,  89,  121. 
Mercury,  or  quicksilver,  149. 
^feridians,  100. 
^Microscope,  21 1,  214. 
,  sins^'le,  :^11. 

» solar,  211. 

Minerals,  16. 
Minutes,  101. 
i^  Lou  soon  s,  157. 
Month,  lunar,  116. 
Momcntiini,  43.  63. 
Moon,  80,  87,  91,  It B,  121. 


I^'DEX. 


255 


Moon -light,  171 
Motion,  18,58,44,  45. 
— — ,  uniform,  40. 

,  perpetual,  40, 

— — ,  retarded,  41. 

,  accelerated,  41 . 

— — ,  reflected,  48. 
— — ,  compound,  52. 
— — ,  circular,  53,  70. 
,  axis  of,  54,  61, 

— ,  centre  of,  54,  60,  122. 

— — — ,  diurnal,  84. 
Musical  instruments,  164. 
Mirrors,  183. 
— — — ,  reflection  of,  183. 

,  plane  or  flat,  185, 

,  convex,  185. 
■    ■     -,  concave,  185,  187. 

—- ,  axis  of,  186. 

— ~,  burning,  188. 

N. 

Neap  tides,  124. 
Nerves,  175. 

— — ,  auditory,  164,  175. 
Nerves,  optic,  173,  175. 

— ',  olfactory,  175. 

Night,  84. 

Nodes,  106, 107,  114. 

O. 

Octave,  165, 

Odour,  17. 

Opaque -bodies,  166,  167. 

Optics,  166. 

Orbit,  89. 

P. 

Pallas,  91. 

Parabola,  57. 

Parallel  lines,  30. 

Pellucid  bodies,  167. 

Pencil  of  rays,  167. 

Pendulum,  104. 

Perihelion,  82, 

Perpendicular  lines,  CO,  49,  ICQ. 

Phases,  117. 

Piston,  152. 

Plane,  100. 

Planets,  83,  87,  111. 

Poles,  99. 

Polar  star,  107,  119. 

Porosity,  47. 

Powers,  meclianical,  60. 

Projection,  56,  78. 


Precession  of  the  equinoxes,  114, 
Pulley,  60,  68. 
Pump,  36,  37, 

,  sucking  or  lifting,  1 53, 

I  -,  forcing,  153,  155. 
Pupil  of  the  eye,  173. 

R 

Rain,  138. 
Rainbow,  198. 
Rarity,  20. 
Ray  of  light,  201. 

of  reflection,  169. 

of  incidence,  170. 

Rays,  intersecting,  173. 
Reaction,  44. 
Receiver,  36. 
Reflection  of  light,  169. 

• -,  angle  of,  51,  184. 

of  mirrors,  183. 

— — -  of  plain  mirrors,  1 85. 
-of convex  mirrors,  185. 

of  concave  mirrors,  185. 

Reflected  motion,  48. 
Refraction,  190. 

of  the  atmosphere,  192, 

i of  glass,  194. 

of  a  lens,  194, 

of  a  prism,  195, 

Resistance,  60. 
Retina,  173. 

,  image  on,  174*. 

Rivers,  137. 
Rivulets,  139. 

S. 
Satellites,  87,  1 19,  120. 
Saturn,  91. 
Scales  or  balance,  61. 
Screw,  60,  73. 
Shadow,  117, 168. 
Siderialtime,  113. 
Sight,  175. 

Signs,  Zodiac,  93,  100, 101. 
Smoke,  1 8,  34. 
Solar  Microscope,  211. 
Solstice,  106,  107. 
Sound, 159. 
— — ,  acute,  1 64. 

■  ,  musical,  164. 
Space,  39. 
Specific  £^ravity,  131. 

^ of  air,  149 . 

Spectrum,  196. 
Speaking  trumpet,  163. 
Sphere,  31,  58,  103. 


256 


INDEX. 


SpringSj  137. 
Springtides,  1-24, 
Square,  88,  92. 
Stars,  83,  93,  112,  119. 
Storms,  156. 
Substance,  14, 
Summer,  82,  106. 
Sun,  77,  87,  166,  192. 
Swimming,  46. 
Syphon,  141. 

T. 

Tangent,  55,  79. 
Telescope,  214. 

■  ,  reflecting,  214. 

— ,  refracting,  214. 

Temperate  zone,  100,  lOS. 
Thermometer,  151. 
Tides,  121. 
— -,  neap,  124. 

,  spring,  124. 

■— ,  aerial,  159. 
Time,  112,114. 

,  siderial,  115. 

,  equal,  115. 

■  ,  solar,  115. 
Tone,  164. 

Torrid  zone,  100,  108,  1S6. 
Transparent  bodies,  167. 
Treble  and  bass,  164. 
Tropics,  99. 

U. 

Undulations,  162. 
Unison,  165. 


Y. 
ValTc,  152. 
Vapor,  21,  34,  137. 
Velocitv,  39,  63. 
Venus,  90,  121. 
Vesta,  91. 
Vibration,  161. 
Vision,  178. 

,  angle  of,  178. 

,  double,  182. 

W. 

Waters,  126,  139. 
— — ,  spring,  139. 
-,  rain,  139. 


-,  level  of,  128,  133,  137. 


Wedge,  60,  7?,. 
Weight,  20,  28,103, 131, 146,  147. 
Wheel  and  axle,  60,  71. 
Wind,  155. 

,  trade,  156. 

,  periodical,  157, 

Winch,  73. 
Winter,  82,  107. 


Year,  112. 

,  siderial,  113. 

— ,  solar,  114. 

Z. 

Zodiac,  93. 
Zone,  100. 

,  torrid,  100,  108,  156, 193. 

,  temperate,  100,  108. 

,  frigid,  100,  107. 


CONTENTS. 

CONVERSATION  I. 

On  General  Properties  of  Bodies, 

Introdwction  ;  General  Properties  of  Bodies  ;  Impenetrability  ; 
Extension  ;  Figure  ;  Divisibility  ;  Inertia  ;  Attraction  ;  Attraction 
of  Cohesion;  Density;  Rarity;  Heat;  Attraction  of  Gravitation. 

Page  13 

CONVERSATION  II. 

On  the  Attraction  of  Gravity. 

Attraction  of  Gravitation,  continued  ;  Of  Weight  ;  Of  the  fall  of 
Bodies  ;  Of  the  resistance  of  the  Air  ;  Of  the  Ascent  of  Light  Bodies. 

Page  27 

CONVERSATION  HI. 

On  the  Laws  of  Motion. 

Of  Motion;  Of  the  Inertia  of  Bodies;  Of  Force  to  Produce  Motion; 
Direction  of  Motion  ;  Velocity,  absolute  and  relative ;  Uniform 
Motion  ;  Retarded  Motion  ;  Accelerated  Motion;  Velocity  of  Fall- 
ing Bodies  ;  Momentum  ;  Action  and  Re-action  equal ;  Elasticity 
of  Bodies;  Porosity  of  Bodies;  Reflected  Motion;  Angles  of  Inci- 
dence ai}d  Pwcflection.  Page  3$ 

CONVERSATION  IV. 

On  Compound  Motion. 

Compound  Motion  the  result  of  two  opposite  forces  ;  Of  Circular  Mo- 
tion, the  »-esuit  of  two  forces,  one  of  which  confines  the  body  to  a 
fixed  point  ;  Centre  of  motion,  the  point  at  rest  while  the  other  parts 
of  the  body  move  round  it;  Centre  of  Magnitude,  the  middle  of  a 
body  ;  Centripetal  Force,  that  which  confines  a  body  to  a  fixed 
central  point ;  Centrifugal  Force,  that  which  impels  a  body  to  fly 
from  tlie  centre  ;  Fall  of  Bodies  in  a  Parabola  ;  Centre  of  Gravity, 
the  Centre  of  W^eight,  or  point  about  which  the  parts  balance  each 
other.  Page  52 


\J  CONTENTS. 

CONVERSATION  V. 

On  the  Mechanical  Powers, 

Of  the  Power  of  Machines  ;  Of  the  Lever  in  general;  Of  the  Lever  of 
the  first  kind,  having  the  Fulcrum  between  the  Power  and  the  weight ; 
Of  the  Lever  of  the  second  kind,  having  the  weight  between  the  power 
and  the  Fulcrum;  Of  the  Lever  of  the  third  kind,  having  the  power 
between  the  Fulcrum  and  the  Weight;  Of  the  Pulley  ;  Of  the  Wheel 
and  Axle  ;  Of  the  inclined  Plane  ;  Of  the  Wedge  ,  Of  the  Screw. 

Page  60 

CONVERSATION  VI. 

ASTRONOMY. 

Causes  of  the  Earth^s  Annual  Motion. 

©f  the  Planets,  and  their  motion  ;  Of  the  Diurnal  motion  of  the  Earth 
and  Planets.  Page  77 

CONVERSATION  VII. 

On  the  Planets, 

Of  the  Satellites  or  Moons;  Gravity  diminishes  as  the  Square  of  the 
Distance  ;  Of  the  Solar  System  ;  Of  Comets;  Constellations,  signs  of 
the  Zodiac  ;  Of  Copernicus,  Newton,  Szc.  Page  87 

CONVERSATION  VIII. 

On  the  Earth. 

Of  the  Terrestrial  Globe  ;  Of  the  Figure  of  the  Earth  ;  Of  the  Pendulum ; 
Of  the  Variation  of  the  Seasons,  and  of  the  Length  of  Days  and 
Nights  ;  Of  the  Causes  of  the  Heat  of  Summer  ;  Of  Solar,  Siderial, 
and  Equal  or  Mean  Time.  Page  9S 

CONVERSATION  IX. 

On  the  Moon. 

Of  the  Moon's  Motion  ;  Phases  of  the  Moon  ;  Eclipses  of  the  Moon  ; 
Eclipsesof  Jupiter's  Moons  ;  Of  the  Latitude  and  Longitude  ;  Of  the. 
transits  of  the  inferior  Planets  ;  Of  the  Tides.  Page  116 


CONTENTS*  Vll 

CONVERSATION  X. 

HYDROSTATICS. 
On  the  Mechanical  Properties  of  Fluids, 

Definition  of  a  fluid  ;  Distinction  between  Fluids  and  Liquids  ;  of  Non- 
Elastic  Fluids,  scarcely  susceptible  of  Compression  ;  Of  the  Cohesion 
of  Fluids  ;  Of  their  Gravitation  ;  Of  their  Equilibrium  ;  Of  their 
Pressure  ;  Of  Specific  Gravity  ;  Of  the  Specific  Gravity  of  Bodies 
heavier  than  Water  ;  Of  those  of  the  same  weight  as  water  ;  Of  those 
lighter  than  Water  ;  Of  the  Specific  Gravity  of  Fluids.  Page  126 

CONVERSATION  XL 

Of  Springs,  Fountains,  ^c. 

Of  the  Ascent  of  Vapor  and  the  Formation  of  Clouds  ;  Of  the  Formation 
and  Fall  of  Rain,  kc. ;  Of  the  Formation  of  Springs  ;  Of  Rivers  as.d 
Lakes  ;  Of  Fountains.  Page  137 

CONVERSATION  XII. 

PNEUMATICS. 
On  the  Mechanical  Properties  of  Air. 

Of  the  Spring  or  Elasticity  of  the  Air ;  Of  the  Weight  of  the  Air ; 
Experiments  with  the  Air  Pump  ;  Of  the  Barometer ;  Mode  of 
Weighing  Air;  Specific  Gravity  of  Air  ;  Of  Pumps  ;  Description  of  the 
Sucking  Pump;  Description  of  the  Forcing  Pump.  Page  14S 

CONVERSATION  XIIL 

On  Wind  and  Sound. 

Of  Wind  in  General  ;  Of  the  Trade  Wind;  Of  the  Periodical  Trade 
Winds;  Of  the  Aerial  Tides;  Of  Sound  in  General;  Of  Sonorous 
Bodies  ;  Of  Musical  Sounds  ;  Of  Concord  or  Harmony,  and  Melody. 

Page  155 

CONVERSATION  XIV. 

On  Optics. 

Of  Luminous,  Transparent,  and  Opaque  Bodies;  Of  the  Radiation  of 
Light;  Of  Shadows  ;  Of  the  Reflection  of  Light ;  Opaque  Bodies  seen 
only  by  Reflected  Light ;  Vision  Explained  ;  Camera  Obscura ; 
Image  of  Objects  on  the  Retina.  Page  166 


viii  CONTENTS. 

CONVERSATION  XV. 

On  the  Angle  of  Vision^  and  Reflection  of  Mirrors. 

Angle  of  Vision  ;  Reflection  of  Plain  Mirrors  ;  Reflection  of  Convex 
Mirrors  ;  Reflection  of  Concave  Mirrors,  Page  178^ 

CONVERSATION  XVI. 

On  Refraction  and  Colors. 

Transmission  of  Light  by  Transparent  Bodies  ;  Refraction  ;  Refraction 
of  the  Atmosphere;  Refraction  of  a  Lens;  Refraction  of  the  Prism; 
Of  the  Colors  of  Rays  of  Light ;  Of  the  Colors  of  Bodies.         Page  190 

CONVERSATION  XVIL 

OPTICS. 
On  the  Stimcture  of  the  Eye^  and  Optical  Instruments. 

Description  of  the  Eye  ;  Of  the  Image  on  the  Retina  ;  Refraction  of  the 
Humors  of  the  Eye;  Of  the  Use  of  Spectacles ;  Of  the  Single  Micro- 
scope ;  Of  the  Double  tVTicroscope  ;  Of  the  Solar  Microscope  ;  Magic 
Lanthorn  ;  Refracting  Telebcope;  Reflecting  Telescope. 

Page  205 


CON  VEHSATION  1. 


ON  GENERAL  PROPERTIES  OF  BODIES. 

Introduction  ;  General  Properties  of  Bodies  ;  Impenetr abil- 
ity ;  Extension  ;  Figure  ;  Divisihilitij  ;  Inertia  ;  At- 
traction ;  Attraction  of  Cohesion  ;  Density;  liarity  ; 
Heat  ;  Attraction  of  Gravitation. 


EMILY. 

I  MUST  request  your  assistance^  my  dear  Mrs.  B.  in  a  charge 
which  I  have  lately  undertaken  ;  it  is  that  of  instructing  my 
youngest  sister,  a  task,  which  I  find  proves  more  difficult^ than 
I  had  at  first  imagined.  1  can  teach  her  the  common  routine 
of  children's  lessons  tolerably  well  ;  but  she  is  such  an  inquis- 
itive little  creature,  that  she  is  not  satisfied  without  an  expla- 
nation of  every  difficulty  that  occurs  to  her,  and  frequently 
asks  me  questions  which  I  am  at  a  loss  to  answer.  This 
morning,  for  instance,  when  I  had  explained  to  her  that  the 
%vorld  w^as  round  like  a  ball,  instead  of  being  fiat  as  she  had 
supposed,  and  that  it  w^as  surrounded  by  the  air,  she  asked 
me  what  supported  it.  I  told  her  that  it  required  no  support ; 
she  then  enquired  why  it  did  not  fall  as  every  thing  else  did  ? 
This  I  confess  perplexed  me  ;  for  I  had  myself  been  satisfied 
w^ith  learning  that  the  world  floated  in  the  air,  without  consid- 
ering how  unnatural  it  was  that  so  heavy  a  body,  bearing  the 
weight  of  all  otli^r  things,  should  be  able  to  support  itself. 

Mrs.  B.  1  make  no  doubt,  my  dear,  but  that  I  shall  be 
able  to  explain  this  difficulty  to  you ;  but  1  believe  that  it 
would  be  almost  impossible  to  render  it  intelligible  to  the 
comprehension  of  so  young  a  child  as  your  sister  Sophia. 
You,  who  are  now  in  your  thirteenth  year,  may,  I  think,  witli 
great  propriety,  learn  not  only  the  cause  of  this  partictdar 
2 


? 


14  GENERAL  PROPEKTIES  OP  BODIES. 

fact,  but  acquire  a  general  knowledge  of  the  laws  by  which  the 
natural  world  is  governed. 

Emily,  Of  all  things  it  is  what  I  should  most  like  to  learn ; 
but  I  was  afraid  it  was  too  difficult  a  study  even  at  my  age. 

Mrs,  B,  Not  when  familiarly  explained  ;  if  you  have 
patience  to  attend,  I  will  most  willingly  give  you  all  the  infor- 
mation in  my  power.  You  may  perhaps  find  the  subject 
rather  dry  at  first ;  but  if  I  succeed  in  explaining  the  laws  of 
nature,  so  as  to  make  you  understand  them,  I  am  sure  that 
you  will  derive  not  only  instruction,  but  great  amusement 
from  that  study. 

Emily,  I  make  no  doubt  of  it,  Mrs.  B. ;  and  pray  begin 
by  explaining  why  the  earth  requires  no  support ;  for  that  is 
the  point  which  just  now  most  strongly  excites  my  curiosity. 

I^irs,  B,  My  dear  Emily,  if  I  am  to  attempt  to  give  you  a 
general  idea  of  the  laws  of  nature,  which  is  no  less  than  to 
introduce  you  to  a  knowledge  of  the  science  of  natural  philos- 
ophy, it  will  be  necessary  for  us  to  proceed  with  some  degree 
of  regularity .  I  do  not  wish  to  confine  you  to  the  systematic 
order  of  a  scientific  treatise  ;  but  if  we  were  merely  to  exam- 
ine every  vague  question  thai  may  chance  to  occur,  our  pro- 
gress would  be  but  very  slow.  Let  us,  therefore,  begin  by 
taking  a  short  survey  of  the  general  properties  of  bodies, 
some  of  which  must  necessarily  be  explained  before  I  can 
attempt  to  make  you  understand  why  the  earth  requires  no 
support. 

When  I  speak  of  bodies,  I  mean  substances,  of  whatever 
nature^  whether  solid  or  fluid  ;  and  matte?'  is  the  general  term 
used  to  denote  the  substance,  w  hatever  its  nature  be,  of  which 
the  difierent  bodies  are  composed.  Thus,  wood  is  the  matter 
of  which  this  table  is  made  ;  water  is  the  matter  with  whicli 
this  glass  is  filled,  &c. 

Emily,  I  am  very  glad  you  have  explained  the  meaning 
of  the  word  matter,  as  it  has  corrected  an  erroneous  concep- 
tion I  had  formed  of  it :  I  thought  that  it  w^as  applicable  to 
solid  bodies  only. 

Mrs,  B,  There  are  certain  properties  which  appear  to  be 
common  to  all  bodies,  and  are  hence  called  the  essential 
properties  of  bodies  ;  these  are.  Impenetrability,  Extension, 
Figure,  Divisibility,  Inertia,  and  Attraction,  These  are 
called  the  general  properties  of  bodies,  as  we  do  not  suppose 
any  body  to  exist  without  them. 

By  impenetrability y  is  meant  the  property  which  bodies 


GENERAL  PROPERTIES  OF  BODIES.  15 

have  of  occupying  a  certain  space,  so  that,  where  one  body  is, 
another  cannot  be,  without  displacing  the  former  ;  for  two 
bodies  cannot  exist  in  the  same  place  at  the  same  time.  A 
liquid  may  be  more  easily  removed  than  a  solid  body  ;  yet  it 
is  not  the  less  substantial  since  it  is  as  impossible  for  a  liquid 
and  a  sohd  to  occupy  the  same  space  at  the  same  time,  as  for 
two  solid  bodies  to  do  so.  For  instance,  if  you  put  a  spoon 
into  a  glass  full  of  water,  the  water  will  flow  over  to  make 
room  for  the  spoon. 

Emily,  I  understand  this  perfectly.  Liquids  are  in  reality 
as  substantial  or  as  impenetrable  as  solid  bodies,  and  they 
appear  less  so,  only  because  they  are  more  easily  displaced. 

Mrs,  B,  The  air  is  a  fluid  differing  in  its  nature  from 
liquids,  but  no  less  impenetrable.  If  I  endeavour,  to  fill  this 
phial  by  plunging  it  into  this  bason  of  water,  the  air,  you  see, 
rushes  out  of  the  phial  in  bubbles,  in  order  to  make  way  for 
the  water,  for  the  air  and  the  water  cannot  exist  together  in  the 
same  space,  any  more  than  two  hard  bodies  ;  and  if  I  reverse 
this  goblet,  and  plunge  it  perpendicularly  into  the  water,  so 
that  the  air  will  not  be  able  to  escape,  the  water  will  no  longer 
be  able  to  fill  the  goblet. 

Emily,     But  it  rises  a  considerable  way  into  the  glass. 

Mrii.  B.  Because  the  water  compresses  or  squeezes  the  air 
into  a  small  space  in  the  upper  part  of  the  glass  ;  but,  as  long 
as  it  remains  there,  no  other  body  can  occupy  the  same  place. 

Emily,  A  difficulty  has  just  occurred  to  me,  with  regard  to 
the  impenetrability  of  solid  bodies  ;  if  a  nail  is  driven  into  a 
piece  of  wood,  it  penetrates  it,  and  both  the  wood  and  the  nail 
occupy  the  same  space  that  the  wood  alone  did  before  ? 

Mrs,  B,  The  nail  penetrates  between  the  particles  of  the 
wood,  by  forcing  them  to  make  way  for  it ;  for  you  know  that 
not  a  single  atom  of  wood  can  remain  in  the  space  which  the 
nail  occupies  ;  and  if  the  wood  is  not  increased  in  size  by  the 
addition  of  the  nail,  it  is  because  wood  is  a  porous  substance, 
like  sponge,  the  particles  of  which  may  be  compressed  or 
squeezed  closer  together  ;  and  it  is  thus  that  they  make  way 
for  the  nail. 

We  may  now  proceed  to  the  next  general  property  of  bodies, 
extension,  A  body  which  occupies  a  certain  space  must 
necessarily  have  extension ;  that  is  to  say,  lengthy  breadth^ 
and  de])tli ;  these  are  called  the  dimensions  of  extension  ;  can 
you  form  an  idea  of  any  body  without  them  ? 

Emily,     No ;  certainly  I  cannot ;  though  these  dimensions 


lO  GENERAL  PROPERTIES  OF  BODIES. 

must,  of  course,  vary  extremely  in  different  bodies.  The 
length,  breadth,  and  depth,  of  a  box,  or  of  a  thimble,  are 
very  different  from  those  of  a  walking-stick,  or  of  a  hair. 

But  is  not  height  also  a  dimension  of  extension  ? 

Mrs,  B.  Height  and  depth  are  the  same  dimension,  con- 
sidered in  different  points  of  view  ;  if  you  measure  a  body,  or 
a  space,  from  the  top  to  the  bottom,  you  call  it  depth  ;  if  from 
1  he  bottom  upwards,  you  call  it  height ;  thus  the  depth  and 
height  of  a  box  are,  in  fact,  the  same  thing. 

Emih/.  Very  true  ;  a  moment's  consideration  would  have 
enabled  me  to  discover  that ;  and  breadth  and  width  are  also 
the  same  dimension. 

Mrs,  B,  Yes  ;  the  limits  of  extension  constitute^,e?/^*e  or 
shape.  You  conceive  that  a  body  having  length,  breadth,  and 
depth,  cannot  be  without  form,  either  symmetrical  or  irregular  ? 

Emily.  Undoubtedly  ;  and  this  property  admits  of  almost 
an  infinite  variety. 

ii/rs.  B.  Nature  has  assigned  regular  forms  to  her  pro- 
ductions in  general.  The  natural  form  of  mineral  substances 
is  that  of  crystals,  of  which  there  is  a  great  variety.  Many 
of  them  are  very  beautiful,  and  no  less  remarkable  by  their 
transparency,  or  colour,  than  by  the  perfect  regularity  of  their 
forn^Sj-as  mav  be  seen  in  the  various  museums  and  collections 
of  natural  history.  The  vegetable  and  animal  creation  appears 
less  symmetrical,  but  is  still  more  diversified  in  figure  than  the 
mineral  kingdom.  Manufactured  substances  assume  the  vari- 
ous arbitrary  forms  which  the  art  of  man  designs  for  them  ; 
and  an  infinite  number  of  irregular  forms  are  produced  by 
fractures,  and  by  the  dismemberment  of  the  parts  of  bodies. 

Emily.     Such  as  a  piece  of  broken  china  or  glass  ? 

Mrs.  B.  Or  the  fragments  of  mineral  bodies  which  are 
broken  in  being  dug  out  of  the  earth,  or  decayed  by  the  effect 
of  torrents  and  other  causes.  The  picturesque  efiect  of  rock- 
scenery  is  in  a  great  measure  owing  to  accidental  irregidari- 
ties  of  this  kind. 

We  may  now  proceed  to  divisihility  ;  that  is  to  say,  a 
susceptibility  of  being  divided  into  an  indefinite  number  of 
parts.  Take  any  small  quantity  of  matter,  a  grain  of  sand 
for  instance,  and  cut  it  into  two  parts  ;  these  two  parts  might 
be  again  divided,  had  we  instruments  sufficient!}'-  fine  for  the 
purpose  ;  and  if,  by  means  of  pounding,  grinding,  and  other 
similar  methods,  we  carry  this  division  to  the  greatest  possible 
extent,  and  reduce  the  body  to  its  finest  imaginable  particles^ 


GENERAL  PROPERTIES  OF  BODIES.  .  1/ 

yet  not  one  of  the  particles  will  be  destroyed,  and  the  body 
will  continue  to  exist,  though  in  this  altered  state. 

The  melting  of  a  solid  body  in  a  liquid  affords  a  very  strik- 
ing example  of  the  extreme  divisibility  of  matter  ;  when  you 
sweeten  a  cup  of  tea,  for  instance,  with  what  minuteness  the 
sugar  must  be  divided  to  be  diffused  throughout  the  whole  of 
the  liquid. 

Emily,  And  if  you  pour  a  few  drops  of  red  wine  into  a. 
glass  of  water,  they  immediately  tinge  the  whole  of  the  water, 
and  must  therefore  be  diffused  throughout  it. 

Mrs.  B,  Exactly  so ;  and  the  perfume  of  this  lavender 
water  will  be  almost  as  instantaneously  diffused  throughout 
the  room,  if  I  take  out  the  stopper. 

Emily,  But  in  this  case  it  is  only  the  perfume  of  the  lav- 
ender, and  not  the  water  itself,  that  is  diffused  in  the  room  ? 

Mrs,  B,  The  odour  or  smell  of  a  body  is  part  of  the  body 
itself,  and  is  produced  by  very  minute  particles  or  exhalations 
which  escape  from  odoriferous  bodies.  It  would  be  impossi- 
ble that  you  should  smell  the  lavender  water,  if  particles  of  it 
did  not  come  in  actual  contact  with  your  nose. 

Emily,  But  when  I  smell  a  flower,  I  see  no  vapour  rise 
from  it ;  and  yet  I  can  perceive  the  smell  at  a  considerable 
distance. 

Mrs,  B,  You  could,  I  assure  you,  no  more  smell  a  flower, 
the  odoriferous  particles  of  which  did  not  touch  your  nose, 
than  you  could  taste  a  fruit,  the  flavoured  particles  of  which 
did  not  come  in  contact  with  your  tongue. 

Emily.  That  is  wonderful  indeed ;  the  particles  then, 
which  exhale  from  the  flower  and  from  the  lavender  water, 
are,  I  suppose,  too  small  to  be  visible  ? 

Mrs,  B,  Certainly  :  you  may  form  some  idea  of  their 
extreme  minuteness  from  the  immense  number  which  must 
have  escaped  in  order  to  perfume  the  whole  room  ;  and  yet 
there  is  no  sensible  diminution  of  the  liquid  in  the  phial. 

Emily.     But  the  quantity  must  really  be  diminished  ? 

Mrs.  B.  Undoubtedly  ;  and  were  you  to  leave  the  bottle 
open  a  sufficient  length  of  time,  the  whole  of  the  water  would 
evaporate  and  disappear.  But  though  so  minutely  subdivided 
as  to  be  imperceptible  to  any  of  our  senses,  each  particle 
would  continue  to  exist ;  for  it  is  not  within  the  power  of 
man  to  destroy  a  single  particle  of  matter  :  nor  is  there  any 
reason  to  suppose  that  in  nature  an  atom  is  ever  annihilated, 

Emily,     Yet,  when  a  body  is  burnt  to  ashes,  part  of  it,  at 

a* 


18  GENERAL  PROPERTIES  OF  BODIES. 

leastj  appears  to  be  efTectiially  destroyed  ?  Look  how  small 
is  the  residue  of  ashes  beneath  the  grate,  from  all  the  coals 
which  have  been  consumed  within  it. 

Mrs,  B.  That  part  of  the  coals,  which  you  suppose  to  be 
destroyed,  evaporates  in  the  form  of  smoke  and  vapour,  whilst 
the  remainder  is  reduced  to  ashes.  A  body,  in  burning, 
undergoes  no  doubt  very  remarkable  changes  ;  it  is  generally 
subdivided  ;  its  form  and  colour  altered  ;  its  extension  in- 
creased ;  but  the  various  parts,  into  which  it  has  been  sepa- 
rated by  combustion,  continue  in  existence,  and  retain  all  the 
essential  properties  of  bodies. 

Emily,  But  that  part  of  a  burnt  body  which  evaporates 
in  smoke  has  no  figure  ;  smoke,  it  is  true,  ascends  in  columns 
into  the  air,  but  it  is  soon  so  much  diffused  as  to  lose  all  form  ; 
it  becomes  indeed  invisible. 

Mrs.  B.  Invisible,  I  allow  ;  but  we  must  not  imagine  that 
what  we  no  longer  see  no  longer  exists. — Were  every  particle 
of  matter  that  becomes  invisible  annihilated,  the  world  itself 
would  in  the  course  of  time  be  destroyed.  The  particles  of 
smoke,  when  diffused  in  the  air,  continue  still  to  be  particles 
of  matter,  as  well  as  when  more  closely  united  in  the  form  of 
coals  :  they  are  really  as  substantial  in  the  one  state  as  in  the 
other,  and  equally  so  when  by  their  extreme  subdivision  they 
become  invisible.  No  particle  of  matter  is  ever  destroyed  : 
this  is  a  principle  you  must  constantly  remember.  Every 
thing  in  nature  decays  and  corrupts  in  the  lapse  of  time. 
We  die,  and  our  bodies  moulder  to  dust ;  but  not  a  single 
atom  of  them  is  lost ;  they  serve  to  nourish  the  earth,  whence, 
while  living,  they  drew  their  support. 

The  next  essential  property  of  matter  is  called  inertia  ; 
this  word  expresses  the  resistance  which  inactive  matter  makes 
to  a  change  of  state.  Bodies  appear  to  be  equally  incapable 
of  changing  their  actual  state,  whether  it  be  of  motion  or  of 
rest.  You  know  that  it  requires  force  to  put  a  body  which  is 
at  rest  in  motion  ;  an  exertion  of  strength  is  also  requisite  to 
stop  a  body  which  is  already  in  motion.  The  resistance  of 
the  body  to  a  change  of  state,  in  either  case,  is  called  its 
inertia, 

Emily,  In  playing  at  base-ball  I  am  obliged  to  use  all  my 
strength  to  give  a  rapid  motion  to  the  ball ;  and  when  I  have 
to  catch  it,  I  am  sure  I  feel  the  resistance  it  makes  to  being 
stopped.  But  if  I  did  not  catch  it,  it  wodd  soon  fall  to  the 
ground  and  stop  of  itself. 


GENERAL  PROPERTIES  OF  BODIES.  19 

Mrs.  B,  Inert  matter  is  as  incapable  of  stopping  of  itself, 
as  it  is  of  putting  itself  into  motion  :  when  the  ball  ceases  to 
move,  therefore,  it  must  be  stopped  by  some  other  cause  or 
povrer ;  but  as  it  is  one  with  which  you  are  yet  unacquainted, 
we  cannot  at  present  investigate  its  effects. 

The  last  property  which  appears  to  be  common  to  all  bodies 
is  attraction.  All  bodies  consist  of  infinitely  small  particles 
of  matter,  each  of  which  possesses  the  powder  of  attracting  or 
draw^ing  towards  it,  and  uniting  whh  any  other  particle  sufti- 
eiently  near  to  be  within  the  influence  of  its  attraction  ;  but 
in  minute  particles  this  power  extends  to  so  very  small  a 
distance  around  them  that  its  effect  is  not  sensible,  unless  they 
are  (or  at  least  appear  to  be)  in  contact ;  it  then  makes  them 
stick  or  adhere  together,  and  is  hence  called  the  attraction  of 
cohesion.  Without  this  power,  solid  bodies  would  fall  in 
pieces  or  rather  crumble  to  atoms. 

Emily,  I  am  so  much  accustomed  to  see  bodies  firm  and 
solid,  that  it  never  occurred  to  me  that  any  power  was  requi- 
site to  unite  the  particles  of  which  they  are  composed.  But 
the  attraction  of  cohesion  does  not,  I  suppose,  exist  in  liquids  ; 
for  the  particles  of  liquids  do  not  remain  together  so  as  to  form 
a  body,  unless  confined  in  a  vessel  ? 

Mrs,  B,  I  beg  your  pardon  ;  it  is  the  attraction  of  cohe- 
sion which  holds  this  drop  of  Avater  suspended  at  the  end  of 
my  finger,  and  keeps  the  minute  watery  particles  of  which  it  is 
composed  united.  But  as  this  power  is  stronger  in  proportion 
as  th3  particles  of  bodies  are  more  closely  united,  the  cohesive 
attraction  of  solid  bodies  is  much  greater  than  that  of  fluids. 

The  thmner  and  lighter  a  fluid  is,  the  less  is  the  cohesive 
attraction  of  its  particles,  because  they  are  further  apart ;  and 
in  elastic  fluids,  such  as  air,  there  is  no  cohesive  attraction 
among  the  particles. 

Emily,  That  is  very  fortunate  ;  for  it  would  be  impossible 
to  breathe  the  air  in  a  solid  mass  ;  or  even  in  a  liquid  state. 

But  is  the  air  a  body  of  the  same  nature  as  other  bodies  ? 

Mrs,  B,     Undoubtedly,  in  all  essential  properties. 

Emily,  Yet  you  say  that  it  does  not  possess  one  of  the 
general  properties  of  bodies — cohesive  attraction  ? 

Mrs,  B,  The  particles  of  air  are  not  destitute  of  the  power 
of  attraction,  but  they  are  too  far  distant  from  each  other  to 
be  influenced  by  it ;  and  the  utmost  eflbrts  of  human  art  have 
proved  ineffectual  in  the  attempt  to  compress  them,  so  as  tp 
bring  them  within  the  sphere  of  each  other's  attraction,  and 
make  them  cohere. 


20  GKNERAL  PROPERTIES  OF  BODIES. 

Emily.  Ifso,  howisit  possible  to  prove  that  they  are 
endowed  with  this  power  ? 

Mrs.  B.  The  air  is  formed  of  particles  precisely  of  the 
same  nature  as  those  which  enter  into  the  composition  of 
liquid  and  solid  bodies,  in  which  state  we  have  a  proof  of  their 
attraction. 

Emily .  It  is  then,  I  suppose,  owing  to  the  different  degrees 
of  attraction  of  different  substances,  that  they  are  hard  or  soft ; 
and  that  liquids  are  thick  or  thin  ? 

Mrs.  B.  Yes  ;  but  you  would  express  your  meaning  better 
by  the  term  dcnsify^  which  denotes  the  degree  of  closeness 
and  compactness  of  the  particles  of  a  body  :  thus  you  may 
say,  both  of  solids,  and  of  liquids,  that  the  stronger  the  cohe- 
sive attraction  the  greater  is  the  density  of  the  body.  In  phi- 
losophical language,  density  is  said  to  be  that  property  of 
bodies  by  which  they  contain  a  certain  quantity  of  matter, 
under  a  certain  bulk  or  magnitude.  Rarity  is  the  contrary 
of  density  ;  it  denotes  the  thinness  and  subtlety  of  bodies  : 
thus  you  would  say  that  mercury  or  quicksilver  was  a  very 
dense  fluid  ;  ether,  a  very  rare  one,  &c. 

Caroline.  But  how  are  we  to  judge  of  the  quantity  of 
matter  contained  in  a  certian  bulk  ? 

Mrs.  B.  By  the  weight :  under  the  same  bulk  bodies  are 
said  to  be  dense  in  proportion  as  they  are  heavy. 

Emily.  Then  we  may  say  that  metals  are  dense  bodies, 
wood  comparatively  a  rare  one,  &c.  But,  Mrs.  B.,  when  the 
particles  of  a  body  are  so  near  as  to  attract  each  other,  the 
effect  of  this  power  must  increase  as  they  are  brought  by  it 
closer  together  ;  so  that  one  would  suppose  that  the  body 
would  gradually  augment  in  density,  till  it  was  impossible  for 
its  particles  to  be  more  closely  united.  Now,  we  know  that 
this  is  not  the  case  ;  for  soft  bodies,  such  as  cork,  sponge,  or 
butter,  never  become,  in  consequence  of  the  increasing  attrac- 
tion of  their  particles,  as  hard  as  iron  ? 

Mrs.  B.  In  such  bodies  as  cork  and  sponge,  the  particles 
which  come  in  contact  are  so  few  as  to  produce  but  a  slight 
degree  of  cohesion  :  they  are  porous  bodies,  which,  owing  to 
the  peculiar  arrangement  of  their  particles,  abound  with  inters- 
tices which  separate  the  particles  ;  and  these  vacancies  are 
filled  with  air,  the  spring  or  elasticity  of  which  prevents  the 
closer  union  of  the  parts.  But  there  is  another  fluid  much 
more  subtle  than  air,  which  pervades  all  bodies,  this  is  heat. 
Heat  insinuates  itself  more  or  less  between  the  particles  of  all 
bodies,  and  forces  them  asunder ;  you  may  therefore  consider 


i 


OEXERAL  PrvOPERTIES  OF  BODIES.  21 

iit'ai  and  the  attraction  of  cohesion^  as  constantly  acting  in 
opposition  to  each  other. 

Emily.  The  one  endeavounng  to  rend  a  body  to  pieces, 
the  other  to  keep  its  parts  firmly  united. 

Mrs.  B.  And  it  is  this  struggle  between  the  contending 
tbrces  of  heat  and  attraction,  which  prevents  the  extreme 
degree  of  density  which  would  result  from  the  sole  influence 
of  the  attraction  of  cohesion. 

Emihj.  The  more  a  body  is  heated  then,  the  more  its 
particles  will  be  separated. 

Mrs.  B.  Certainly  ;  we  find  that  bodies  swell  or  dilate 
hy  heat :  this  effect,  is  very  sensible  in  butter,  for  instance, 
which  expands  by  the  application  of  heat,  till  at  length  the 
attraction  of  cohesion  is  so  far  diminished  that  the  particles 
separate,  and  the  butter  becomes  liquid.  A  similar  effect  is 
produced  by  heat  on  metals,  and  all  bodies  susceptible  of 
being  melted.  Liquids,  you  know,  are  made  to  boil  by  the 
application  of  heat  :  the  attraction  of  cohesion  then  yields 
entirely  to  the  expansive  powder ;  the  particles  are  totally 
separated  and  converted  into  steam  or  vapour.  But  the 
agency  of  heat  is  in  no  body  more  sensible  than  in  air,  which, 
dilates  and  contracts  by  its  increase  or  diminution  in  a  very 
remarkable  degree.* 

Emily.  The  effects  of  heat  appear  to  be  one  of  the  most 
interesting  parts  of  natural  philosophy. 

Mrs.  B.  That  is  true  ;  but  heat  is  so  intimately  connected 
with  chemistry,  that  you  must  allow  me  to  defer  the  investi- 
gation of  its  properties  till  you  become  acquainted  with  that 
science. 

To  return  to  its  antagonist,  the  attraction  of  cohesion  ;  it 
is  this  power  which  restores  to  Vctpour  its  liquid  form,  which 
unites  it  into  drops  when  it  falls  to  the  earth  in  a  shower  of 
rain,  which  gathers  the  dQ\Y  into  brilliant  gems  on  the  blades 
of  grass. 

Emily.     And  I  have  often   observed   that  after  a  shower, 

*  The  expansive  power  of  heat  proiluces  some  of  the  most  iiuerestingf 
phenomena  in  nature.  Tlie  hoiling  of  liquids,  is  the  immediate  result  of 
this  power;  and  the  operation,  although  simple,  is  peculiarly  worthy  of 
notice.  As  the  numerous  particles  hecome  expanded  or  rarified,  they 
are  continually  rising  to,  and  escaping  from  the  surface,  which  occasions 
an  agitation  of  the  liquid,  proportioned,  in  its  violence,  to  the  degree  of 
heat  operating  on  it. — And  on  exposing  our  hands  or  other  limbs  to  the 
fire,  the  internal  fluid  becomes  expanded,  "which  causes  them  to  ap])ear 
swollen  ;  whereas,  when  exposed  to  the  cold,  the  abstraction  of  the  heat 
causes  them  to  be  compressed. 


'22  GENERAL    PROPERTIES  OF  BODIES. 

1  he  water  collects  into  large  drops  on  the  leaves  of  plants  ; 
but  I  cannot  say  that  I  perfectly  understand  how  the  attrac- 
tion of  cohesion  produces  this  effect. 

Mrs,  B,  Rain  does  not  fall  from  the  clouds  in  the  form  of 
drops  J  but  in  that  of  mist  or  vapour,  which  is  composed  of 
very  small  watery  particles  ;  these  in  their  descent,  mutually 
attract  each  other,  and  those  that  are  sufficiently  near  in  con- 
sequence unite  and  form  a  drop,  and  thus  the  mist  is  trans- 
formed into  a  shower.  The  dew  also  was  originally  in  a 
state  of  vapour,  but  is,  by  the  mutual  attraction  of  the 
particles,  formed  into  small  globules  on  the  blades  of  grass  : 
in  a  similar  manner  the  ram  upon  the  leaf  collects  into  large 
drops,  v/hlch  when  they  become  too  heavy  for  the  leaf  to 
support,  fall  to  the  ground. 

Emily.  All  this  is  wonderfully  curious  !  I  am  almost 
bewildered  with  surprise  and  admiration  at  the  number  of  new 
ideas  I  have  already  acquired. 

Mrs,  jB.  Every  step  that  you  advance  in  the  pursuit  of 
natural  science,  will  fill  your  mind  with  admiration  and  grat- 
itude towards  its  Divine  Author.  In  the  study  of  natural 
philosophy,  we  must  consider  ourselves  as  reading  the  book 
of  nature,  in  which  the  bountiful  goodness  and  wisdom  of  God 
is  revealed  to  all  mankind  ;  no  study  can  then  tend  more  to 
purify  the  heart,  and  raise  it  to  a  religious  contemplation  of 
the  Divine  perfections. 

There  is  another  curious  effect  of  the  attraction  of  cohesion 
which  I  must  point  out  to  you.  It  enables  liquids  to  rise 
above  their  level  in  capillary  tubes  ;  these  are  tubes,  the  bores 
of  which  are  so  extremely  small  that  liquids  ascend  within 
them,  from  the  cohesive  attraction  between  the  particles  of 
the  liquid  and  the  interior  surface  of  the  tube.  Do  you 
perceive  the  water  rising  above  its  level  in  this  small  glass 
tube,  which  I  have  immersed  in  a  goblet  full  of  water  ? 

Emihj.  Oh  yes  ;  I  see  it  slowly  creeping  up  the  tube,  but 
now  it  is  stationary  ;  will  it  rise  no  higher  ? 

Mrs.  I>.  No  ;  because  the  cohesive  attraction  between 
the  water  and  the  internal  surface  of  the  tube  is  now  balanced 
by  the  weight  of  the  water  within  it :  if  the  bore  of  the  tube 
were  narrower  the  water  would  rise  higher  ;  and  if  you  im- 
merse several  tubes  of  bores  of  different  sizes,  you  will  see  it 
rise  to  different  heights  in  each  of  them.  In  making  this 
experiment,  you  should  colour  the  water  w  ith  a  little  red  wine, 
in  order  to  render  the  effect  more  obvious. 

All  porous  substances,  such  as  sponge,   bread,  linen,  &c 


GENERAL  PROPERTIES  OF  BODIES.  2S 

may  be  considered  as  collections  of  capillary  tubes  :  if  you 
dip  one  end  of  a  lump  of  sugar  into  water  the  water  will  rise 
in  it ;  and  wet  it  considerably  above  the  surface  of  that  into 
which  you  dip  it. 

"  EmiJt/.     In  making  tea  I  have  often  observed  that  effectj 
without  being  able  to  account  for  it. 

Mrs,  B.  Now  that  you  are  acquainted  with  the  attraction 
of  cohesion,  I  must  endeavor  to  explain  to  you  that  of  Grav- 
itation,  which  is  a  modification  of  the  same  power  ;  the  first 
is  perceptible  only  in  very  minute  particles,  and  at  very  small 
distances  ;  the  other  acts  on  the  largest  bodies,  and  extends 
to  immense  distances. 

Emily.  You  astonish  me  :  surely  you  do  not  mean  to  say 
that  large  bodies  attract  each  other. 

Mrs,  B.  Indeed  I  do  ;  let  us  take,  for  example,  one  of  the 
largest  bodies  in  nature,  and  observe  whether  it  does  not 
attract  other  bodies.  What  is  it  that  occasions  the  fall  of  this 
book,  when  I  no  longer  support  it  ? 

Emily,  Can  it  be  the  attraction  of  the  earth  ?  I  thought 
that  all  bodies  had  a  natural  tendency  to  fall. 

Mrs,  B,  They  have  a  natural  tendency  to  fall,  it  is  true  ; 
but  that  tendency  is  produced  entirely  by  the  attraction  of  the 
earth  ;  the  earth  being  so  much  larger  than  any  body  on 
its  surface,  forces  every  body,  wliich  is  not  supported,  to  fall 
upon  it. 

Emily.  If  the  tendency  which  bodies  have  to  fall  results 
from  the  earth's  attractive  power,  the  earth  itself  can  have  no 
such  tendency,  since  it  cannot  attract  itself,  and  therefore  it 
requires  no  support  to  prevent  it  from  frilling.  Yet  the  idea 
that  bodies  do  not  fall  of  their  own  accord,  but  that  they  are 
drawn  towards  the  earth  by  its  attraction,  is  so  new  and 
strange  to  me,  that  I  know  not  how  to  reconcile  myself  to  it. 

Mrs,  B,  When  you  are  accustomed  to  consider  the  fall  of 
bodies  as  depending  on  this  cause,  it  will  appear  to  you  as 
natural,  and  surely  much  more  satisfactory,  than  if  the  cause 
of  their  tendency  to  fall  were  totally  unknown.  Thus  you 
understand,  that  all  matter  is  attractive,  from  the  smallest 
particle  to  the  largest  mass  ;  and  that  bodies  attract  each 
other  with  a  force  proportional  to  the  quantity  of  matter  they 
contain. 

Emily,  I  do  not  perceive  any  difference  between  the 
attraction  of  cohesion  and  that  of  gravitation  :  is  it  not  because 
every  particle  of  matter  is  endowed  with  an  attractive  power, 


24  f.i\St.AAL  rKOPElLiii:.;^  Oi    i;oDiJb.>. 

that  large  bodies^  consisting  of  a  great  number  oi"  paniciesj 
are  so  strongly  attractive  ? 

Mrs.  B.  True.  There  is,  however,  this  difference  be- 
tween the  attraction  of  particles  and  that  of  masses,  that  the 
former  is  stronger  than  the  latter,  in  proportion  to  the  quan- 
tity of  matter.  Of  this  you  have  an  instance  in  the  attraction 
of  capillary  tubes,  in  which  liquids  ascend  by  the  attraction  of 
cohesion,  in  opposition  to  that  of  gravity.  It  is  on  this  ac- 
count that  it  is  necessary  that  the  bore  of  the  tube  should  be 
eitremely  small ;  for  if  the  column  of  water  within  the  tube  is 
not  very  minute,  the  attraction  would  not  be  able  either  to 
raise  or  support  its  weight,  in  opposition  to  that  of  gravity. 

You  may  observe,  also,  that  all  solid  bodies  are  enabled  by 
the  force  of  the  cohesive  attraction  of  their  particles  to  resist 
that  of  gravity,  which  would  otherwise  disunite  them,  and 
bring  them  to  a  level  with  the  ground,  as  it  does  in  the  case 
of  liquids,  the  cohesive  attraction  of  w^iich  is  not  sufficient  to 
enable  it  to  resist  the  powTr  of  gravity.* 

Emily,  And  some  solid  bodies  appear  to  be  of  this  n^l- 
ture,  as  sand  and  powder  for  instance  :  there  is  no  attraction 
of  cohesion  between  their  particles  ? 

*  The  pov/er  of  gravitation  is  greatest  at  the  surface  of  the  earth,  -v^'hence 
it  decreases  both  upwards  and  downwards  ;  but  not  in  the  same  propor- 
tion. The  force  of  gravity  upxvards  is  as  the  square  of  the  distance  from 
the  centre.  That  is,  gravity  at  the  surface  of  the  earth,  which  is  about 
4300  miles  from  the  centre,  is  four  times  more  powerful  than  it  -would 
be  at  double  the  distance,  or  8000  miles  from  tlie  centre.  Gravity  and 
■weight  may  be  taken,  in  particular  circumstances  as  synonymous  terms. 
We  say,  a  piece  of  lead  weighs  a  pound,  or  sixteen  ounces  ;  but  if  by  any 
TAieans  it  could  be  carried  4000  miles  above  the  surface  of  the  earth,  it 
vvould  weigh  only  one  fourth  of  a  pound,  or  four  ounces  ;  and  if  it  could 
be  transported  to  8000  miles  above  the  earth,  which  is  three  times  the 
distance  from  the  centre  that  the  surface  is,  it  would  w^eigh  only  one  ninth 
of  a  pound,  or  something  less  than  two  ounces. 

And  it  is  demonstrated,  that  the  force  of  gravity  downwards  decreases, 
as  the  distance  from  the  surface  increases,  so  that  at  one  half  the  distance 
from  the  centre  to  the  surface,  the  same  weight,  already  described  would 
-weigh  only  one  hail  of  a  pound,  and  so  on. — Thus,  a  x>iece  of  metal  weigh- 
uig,  on  the  surface  of  the  earth,  one  pound,  will 

At  The  centre  weigh     -     -     -     -     0 

1000  miles  from  the  centre       1-4  pound. 

2000 1-2 

5000 3-4 

4000 1 

8000 1-4 

12,000 1-9 

And  at  the  distance  of  the  moon  from  the  earth,  Avhich  is  240,000  miles, 
it  would  weigh  oidy  the  3,600th  part  of  a  pound,  because  the  distance  is 
60  times  further  from  the  centre  of  the  earth  than  the  surface-. 


GE^[ERAL  PKOPERTIES  OF  BODIES.  '      25 

Mrs,  B.  Every  grain  of  powder  or  sand  is  composed  of  a 
great  number  of  other  more  minute  particles,  firmly  united  by 
the  attraction  of  cohesion  ;  but  amongst  the  separate  grains 
tliere  is  no  sensible  attraction,  because  they  are  not  in  suffi- 
ciently close  contact. 

Emily,     Yet  they  actually  touch  each  other  ? 

Mrs,  B.  The  surface  of  bodies  is  in  general  so  rough  and 
uneven,  that  when  in  actual  contact,  they  touch  each  other 
only  by  a  few  points.  Thus,  if  I  lay  upon  the  table  this 
book,  the  binding  of  which  appears  perfectly  smooth,  yet  so 
few  of  the  particles  of  its  under  surface  come  in  contact  with 
the  table,  that  no  sensible  degree  of  cohesive  attraction  takes 
place  ;  for  you  see,  that  it  does  not  stick,  or  cohere  to  the 
table,  and  I  find  no  difficuhy  in  lifting  it  off. 

It  is  only  when  surfaces  perfectly  flat  and  well  polished 
are  placed  in  conta<:t,  that  tlie  particles  approach  in  sufficient 
number,  and  closely  enough,  to  produce  a  sensible  degree  of 
cohesive  attraction.  Here  are  two  hemispheres  of  polished 
metal,  I  press  their  flat  surfaces  together,  having  previously 
interposed  a  few  drops  of  oil,  to  fill  up  every  little  porous 
vacancy.     Now  try  to  separate  them. 

Emily,  It  requires  an  effort  beyond  my  strength,  though 
there  are  handles  for  the  purpose  of  pulling  them  asunder. 
Is  the  firm  adhesion  of  the  two  hemispheres,  merely  owing  to 
the  attraction  of  cohesion  ? 

Mrs,  B,  There  is  no  force  more  powerful,  since  it  is  by 
this  that  the  particles  of  the  hardest  bodies  are  held  together, 
it  would  require  a  weight  of  several  pounds,  to  separate  these 
hemispheres. 

Emily,  In  making  a  kaleidoscope,  I  recollect  that  the  two 
plates  of  glass,  which  were  to  serve  as  mirrors,  stuck  so  fast 
together,  that  I  imagined  some  of  the  gum  I  had  been  using 
had  by  chance  been  interposed  betv/een  them  ;  but  now  I 
make  no  doubt  but  that  it  was  their  own  natural  cohesive 
attraction  which  produced  this  effect. 

Mrs  B,  Very  probably  it  was  so  ;  for  plate-glass  has  an 
extremely  smooth,  flat  surface,  admitting  of  the  contact  of  a 
great  number  of  particles,  between  two  plates,  laid  one  over 
the  other. 

Emily,      But,  Mrs.   B.,  the  cohesive  attraction  of  some 
bodies  is  much  greater  than  that  of  others  ;  thus  glue,  gum, 
and  paste,  cohere  with  singular  tenacity. 
3 


Ju  GENERAL  PROPERTIES  OF  BODIES. 

Mrs,  B.  That  is  owing  to  the  peculiar  chemical  properties 
of  those  bodies,  independently  of  their  cohesive  attraction. 

There  are  some  other  kinds  of  modifications  of  attraction 
peculiar  to  certain  bodies ;  namely,  that  of  magnetism,  and 
of  electricity  ;  but  we  shall  confine  our  attention  merely  to 
the  attraction  of  cohesion  and  of  gravity  ;  the  examination  of 
the  latter  we  shall  resume  at  our  next  meeting. 


CONVERSAnON  II. 


ON  THE  ATTRACTION  OF  GRAVITY. 

Attraction  of  Gramtation,  continued  ;  of  Weight ;  Of  the 
fall  of  Bodies  ;  Of  the  Resistance  of  the  Air  ;  Of  the 
Ascent  of  Light  Bodies, 


EMILY. 

I  HAVE  related  to  my  sister  Caroline  all  that  you  have 
taught  me  of  natural  philosophy,  and  she  has  been  so  much 
delighted  by  it,  that  she  hopes  you  will  have  the  goodness  to 
admit  her  to  your  lessons. 

Mrs.  B,  Very  willingly  ;  but  I  did  not  think  you  had 
any  taste  for  studies  of  this  nature,  Caroline  ? 

Caroline,  I  confess,  Mrs.  B.,  that  hitherto  I  had  formed 
no  very  agreeable  idea,  either  of  philosophy,  or  philosophers  ; 
but  what  Emily  has  told  me,  has  excited  my  curiosity  so 
much,  that  I  shall  be  highly  pleased  if  you  will  allow  me  to 
become  one  of  your  pupils. 

Mrs.  B.  I  fear  that  I  shall  not  find  you  so  tractable  a 
scholar  as  Emily  ;  I  know  that  you  are  much  biassed  in 
favour  of  your  own  opinions. 

Caroline.  Then  you  will  have  the  greater  merit  in  re- 
forming them,  Mrs.  B.  ;  and  after  all  the  wonders  that  Emily 
has  related  to  me,  I  think  I  stand  but  little  chance  against 
you  and  your  attractions. 

Mrs.  B.  You  will,  I  doubt  not,  advance  a  number  of 
objections  ;  but  these  I  shall  willingly  admit,  as  they  will  be 
a  means  of  elucidating  the  subject.  Emily,  do  you  recollect 
the  names  of  the  general  properties  of  bodies  ? 

Emily.  Impenetrability,  extension,  figure,  divisibility, 
inertia,  and  attraction. 


^b  OS  THE  ATTRACTION  OF  GRAVITY. 

Mrs.  B,  Very  well.  You  must  remember  that  these  arf 
properties  common  to  all  bodies,  and  of  which  they  cannot 
be  deprived ;  all  other  properties  of  bodies  are  called  acci- 
ilental,  because  they  depend  on  the  relation  or  connection  of 
one  body  to  another. 

Caroluic,  Yet  surely,  Mrs.  B.,  there  are  other  properties 
which  are  essential  to  bodies,  besides  those  you  have  enumer- 
ated. Colour  and  weight,  for  instance,  are  common  to  ail 
bodies,  and  do  not  arise  from  their  connexion  with  each  other, 
but  exist  in  the  bodies  themselves  ;  these,  therefore,  cannot 
be  accidental  qualities. 

Mrs,  B.  I  beg  your  pardon  ;  these  properties  do  not  exist 
in  bodies  independently  of  their  connection  with  other  bodies. 

Caroline,  What  !  have  bodies  no  weight  ?  Does  not  this 
table  weigh  heavier  than  this  book  ;  and,  if  one  thing  weighs 
iieavier  than  another,  must  there  not  be  such  a  thing  as 
weight  ? 

Mrs,  B,  No  doubt :  but  this  property  does  not  appear  to  be 
essential  to  bodies ;  it  depends  upon  their  connection  with  each 
other.  AVeight  is  an  effect  of  the  power  of  attraction,  without 
which  the  table  and  the  book  would  have  no  weight  whatever. 

Emily.  I  think  I  understand  you  :  is  it  not  the  attraction 
of  gravity,  which  makes  bodies  heavy  ? 

Mrs,  B.  You  are  right.  I  told  you  that  the  attraction  of 
gravity  v/ as  proportioned  to  the  quantity  of  matter  which 
bodies  contained  :  now  the  earth  consisting  of  a  much  greater 
quantity  of  matter  than  any  body  upon  its  surface,  the  force 
of  its  attraction  must  necessarily  be  greatest,  and  must  draw 
every  thing  tovrards  it  ;  in  consequence  of  which,  bodies  that 
are  unsupported  fall  to  the  ground,  w^hilst  those  that  arc  sup- 
[)orted  press  upon  the  object  which  prevents  their  fall,  v/ith  a 
weight  equal  to  the  force  with  which  they  gravitate  towards 
die  earth. 

Caroline,  The  same  cause  then  which  occasions  the  fall 
of  bodies,  produces  also  their  weight.  It  was  very  dull  in  me 
not  to  understand  this  before,  as  it  is  the  natural  and  necessary 
consequence  of  attraction  ;  but  the  idea  that  bodies  were  not 
really  heavy  of  themselves,  appeared  to  me  quite  incompre- 
hensible. But,  Mrs.  B.,  if  attraction  is  a  property  essential  to 
matter,  weight  must  be  so  likewise ;  for  how  can  one  exist 
without  the  other  ? 

Mrs,  B,  Suppose  there  were  but  one  body  existing  in 
universal  space,  what  would  its  weight  be  ? 


ON  THE  ATTRACTION  OF  GRAVITY*  29 

Caroline.  That  would  depend  upon  its  size ;  or,  more 
accurately  speaking,  upon  the  quantity  of  matter  it  contained. 

Emily,  No,  no  ;  the  body  would  have  no  weight  whatever 
were  its  size  ;  because  nothing  would  attract  it.  Am  I  not 
right,  Mrs.  B.  ? 

Mrs.  B.  You  are  :  you  must  allow,  therefore,  that  it  would 
be  possible  for  attraction  to  exist  without  weight ;  for  each  of 
the  particles  of  which  the  body  was  composed,  would  possess 
the  power  of  attraction  ;  but  they  could  exert  it  only  amongst 
themselves  ;  the  whole  mass,  having  nothing  to  attract,  or  to 
be  attracted  by,  would  have  no  weight. 

Caroline.  I  am  now  well  satisfied  that  weight  is  not 
essential  to  the  existence  of  bodies  ;  but  what  have  you  to 
object  to  colours,  Mrs.  B.,  you  will  not,  I  think,  deny  that 
they  really  exist  in  the  bodies  themselves. 

Mrs.  B.  When  we  come  to  treat  of  the  subject  of  colours, 
I  trust  that  I  shall  be  able  to  convince  you,  that  colours  are 
likewise  accidental  qualities,  quite  distinct  from  the  bodies  to 
which  they  appear  to  belong. 

Caroline.  Oh  do  pray  explain  it  to  us  now,  I  am  so  very 
curious  to  know  how  that  is  possible. 

Mrs.  B.  Unless  we  proceed  with  some  degree  of  order 
and  method,  you  will  in  the  end  find  yourself  but  little  the 
wiser  for  all  you  learn.  Let  us  therefore  go  on  regularly,  and 
make  ourselves  well  acquainted  with  the  general  properties 
of  bodies,  before  we  proceed  further. 

Emily.  To  return,  then,  to  attraction,  (which  appears  to 
me  by  far  the  most  interesting  of  them,  since  it  belongs  equal- 
ly to  all  kinds  of  matter,)  it  must  be  mutual  between  two 
bodies  ;  and  if  so,  when  a  stone  falls  to  the  earth,  the  earth 
should  rise  part  of  the  way  to  meet  the  stone  ? 

Mrs.  B.  Certainly  ;  but  you  must  recollect  that  the  force 
of  attraction  is  proportioned  to  the  quantity  of  matter  which 
bodies  contain,  and  if  you  consider  the  difference  there  is  in 
that  respect,  between  a  stone  and  the  earth,  you  will  not  be 
surprised  that  you  do  not  perceive  the  earth  rise  to  meet  the 
stone ;  for  though  it  is  true  that  a  mutual  attraction  takes 
place  between  the  earth  and  the  stone,  that  of  the  latter  is  so 
very  small  in  comparison  to  that  of  the  former,  as  to  render 
its  effect  insensible., 

Emily.  But  since  attraction  is  proportioned  to  the  quantity 
of  matter  which  bodies  contain,  why  do  not  the  hills  attract 
the  houses  aad  churches  towards  them  ? 


30  ON  THE  ATTRACTION  OP  GRAVITY. 

Caroline,  Heavens,  Emily,  what  an  idea  !  Hov/  can  the 
houses  and  churches  be  moved,  when  they  are  so  firmly  fixed 
in  tlie  ground  ? 

Mrs.  B.  Emily's  question  is  not  absurd,  and  your  answer, 
Caroline,  is  perfectly  just ;  but  can  you  tell  us  why  the  houses 
und  churches  are  so  firmly  fixed  in  the  ground  ? 

Caroline,  I  am  afraid  I  have  answered  risfht  by  mere 
chance  ;  for  I  begin  to  suspect  that  bricklayers  and  carpenters 
itould  give  but  little  stability  to  their  buildings,  without  the  aid 
of  attraction. 

Mrs,  B,  It  is  certainly  the  cohesive  attraction  between  the 
bricks  and  the  mortar,  which  enables  them  to  build  walls,  and 
these  are  so  strongly  attracted  by  the  earthy  as  to  resist  every 
other  impulse ;  otherwise  they  would  necessarily  move  towards 
the  hills  and  the  mountains  ;  but  the  lesser  force  must  yield  to 
the  greater.  There  are,  however,  som_e  circumstances  in 
which  the  attraction  of  a  large  body  has  sensibly  counteracted 
that  of  the  earth.  If,  whilst  standing  on  the  declivity  of  a 
mountain,  you  hold  a  plumb-line  in  your  hand,  the  weight  will 
not  fall  perpendicular  to  the  earth,  but  incline  a  httle  towards 
the  mountain  ;  and  this  is  owing  to  the  lateral,  or  sideways 
attraction  of  the  mountain,  interfering  with  the  perpendicular 
attraction,  of  the  earth. 

Emily,  But  the  size  of  a  mountain  is  very  trifling  compared 
ro  the  whole  earth  ? 

Mrs,  B,  Attraction,  you  must  recollect,  diminishes  w^itli 
distance  ;  and  in  the  example  of  the  plumb-line,  the  weight 
suspended  is  considerably  nearer  to  the  mountain  than  to  the 
centre  of  the  earth. 

Caroline,  Pray,  Mrs.  B.,^  do  the  two  scales  of  a  balance 
hang  parallel  to  each  other  ? 

Mrs,  B,  You  mean,  I  suppose,  in  other  words,  to  inquire 
Avhether  two  lines  which  are  perpendicular  to  the  earth,  are 
parallel  to  each  other  ?  I  believe  I  guess  the  reason  of  your 
question  ;  but  I  wish  you  w  ould  endeavour  to  answ^er  it  without 
my  assistance. 

Caroline,  I  was  thinking  that  such  lines  must  both  tend 
by  gravity  to  the  same  point,  the  centre  of  the  earth  ;  now 
lines  tending  to  the  same  point  cannot  be  parallel,  as  parallel 
lines  are  always  at  an  equal  distance  from  each  other,  and 
would  never  meet. 

Mrs,  B,  Very  well  explained ;  you  see  now  the  use  of  your 
knowledge  of  parallel  lines  :  had  you  been  ignorant  of  theio; 


TLATE   I 


ON  THE  ATtRACTION  OF  GP.AVITr.  Si 

properties,  you  couid  not  have  drawn  such  a  conclusion.  This 
may  enable  you  to  form  an  idea  of  the  great  advantage  to  be 
derived  even  from  a  slight  knowledge  of  geometry,  in  the  study 
of  natural  philosophy  ;  and  if,  after  I  have  made  you  acquain- 
ted with  the  first  elements,  you  should  be  tempted  to  pursue 
the  study,  I  would  advise  you  to  prepare  yourselves  by  acqui- 
ring some  knovvdedge  of  geometry.  This  science  v/ould  teach 
you  that  lines  which  fall  perpendicular  to  the  surface  of  a 
sphere  cannot  be  parallel,  because  they  would  all  meet,  if 
prolonged  to  the  centre  of  the  sphere  ;  while  lines  that  fail 
perpendicular  to  a  plane  or  flat  surface,  are  always  parallel, 
because  if  prolonged,  they  would  never  meet. 

Emily,  And  yet  a  pair  of  scales,  hanging  perpendicular 
to  the  earth,  appear  parallel  ? 

Mrs.  B,  Because  the  sphere  is  so  large,  and  the  scales 
consequently  converge  so  little,  that  their  inclination  is  not 
perceptible  to  our  senses  ;  if  we  could  construct  a  pair  of 
scales  whose  beam  would  extend  several  degrees,  their  con- 
vergence would  be  very  obvious  ;  but  as  this  cannot  be  accom- 
plished, let  us  draw  a  small  figure  of  the  earth,  and  then  we 
may  make  a  pair  of  scales  of  the  proportion  we  please, 
(fig.  1.  plate  I.) 

Caroline,  This  figure  renders  it  very  clear  :  then  two 
bodies  cannot  fall  to  the  earth  in  parallel  lines  ? 

Mrs.  B.     Never. 

Caroline,  The  reason  that  a  heavy  body  falls  quicker  than 
a  hght  one,  is,  I  suppose,  because  the  earth  attracts  it  more 
strongly  ? 

Mrs.  B,  The  earth,  it  is  true,  attracts  a  heavy  body  more 
than  a  light  one  ;  but  that  would  not  make  the  one  fall  quicker 
than  the  other. 

Caroline,  Yet,  since  it  is  attraction  that  occasions  the  fall 
of  bodies,  surely  the  more  a  body  is  attracted,  the  more  rapid- 
ly it  will  fall.  Besides,  experience  proves  it  to  be  so.  Do 
we  not  every  day  see  heavy  bodies  fall  quickly,  and  light 
bodies  slowly  ? 

Emily.  It  strikes  me,  as  it  does  Caroline,  that  as  attraction 
is  proportioned  to  the  quantity  of  matter,  the  earth  must 
necessarily  attract  a  body  which  contains  a  great  quantity 
more  strongly,  and  therefore  bring  it  to  the  ground  sooner  than 
one  consisting  of  a  smaller  quantity. 

Mrs,  B.  You  must  consider,  that  if  heavy  bodies  are 
attracted  more  strongly  than  Hght  ones,  they  require  more 


32  ON  THE  ATTRACTION  OF  GRAVITY, 

attraction  to  malie  thcin  fall.  Remember  that  bodies  have  no 
natural  tendency  to  faH,  any  more  than  to  rise,  or  to  move 
laterally,  and  that  they  will  not  fall  unless  impelled  by  some 
force  ;  now  this  force  must  be  proportioned  to  the  quantity  of 
matter  it  has  to  move  :  a  body  consisting  of  1000  particles  of 
matter,  for  instance,  requires  ten  times  as  much  attraction  to 
bring  it  to  the  ground  in  the  same  space  of  time  as  a  body 
consisting  of  only  100  particles. 

Caroline.  I  do  not  understand  that  ;  for  it  seems  to  me, 
that  the  heavier  a  body  is,  the  more  easily  and  readily  it  falls. 

Emily,  I  think  I  nov/  comprehend  it ;  let  me  try  if  I  can 
explain  it  to  Caroline.  Suppose  that  I  draw^  towards  me  tv/o 
weighty  bodies,  the  one  of  lOOlbs.,  the  other  of  lOOOlbs.^  must 
I  not  exert  ten  times  as  much  strength  to  draw  the  larger  one 
to  me,  in  the  same  space  of  time  as  is  required  for  the  smaller 
one  ?  And  if  the  earth  draws  a  body  of  lOOOlbs.  weight  to  it 
in  the  same  space  of  time  that  it  draws  a  body  of  lOOlbs.  does 
li  not  follow  that  it  attracts  the  body  of  lOOOlbs.  weight  with 
ten  times  the  force  that  it  does  that  of  lOOlbs.  ? 

CaroUrie,  I  comprehend  your  reasoning  perfectly  ;  but  if 
it  were  so,  the  body  of  lOOOlbs.  weight,  and  that  of  lOOlbs. 
Vv'ouldfall  with  the  same  rapidity  ;  and  the  consequence  would 
be,  that  all  bodies,  whether  light  or  heavy,  being  at  an  equal 
distance  from  the  ground,  would  fall  to  it  in  the  same  space  of 
time  :  now  it  is  very  evident  that  this  conclusion  is  absurd  ; 
experience  cver}^  instant  contradicts  it  ;  observe  how  much 
sooner  this  book  reaches  the  floor  than  this  sheet  of  paper, 
when  I  let  them  drop  together. 

Emily.  That  is  an  objection  I  cannot  answer.  I  must 
refer  it  to  you,  Mrs.  B. 

Mrs,  B.  I  trust  that  v/e  shall  not  find  it  insurmountable, 
\t  is  true  that,  according  to  the  laws  of  attraction,  all  bodies- 
at  an  equal  distance  from  the  earth,  should  fall  to  it  in  the  same 
space  of  time  ;  and  this  would  actually  take  place  if  no  ob- 
stacle intervened  to  impede  their  fall.  But  bodies  fall  through 
the  air,  and  it  is  the  resistance  of  the  air  which  makes  bodies 
of  different  density  fall  with  different  degrees  of  velocity. 
They  must  all  force  their  way  through  the  air,  but  dense  heavy- 
bodies  overcome  this  obstacle  more  easily  than  rarer  and 
lighter  ones> 

The  resistance  which  the  air  opposes  to  the  fall  of  bodies 
is  proportioned  to  their  surface,  not  to  their  weight ;  the  air 
b^ing  inert^  cannot  exert  a  greater  foice  to  support  the  weight 


ON  THE  ATTE ACTION  OF  GRAVITY.  SS 

of  a  cannon-ballj  than  it  does  to  support  the  weight  of  a  ball 
(of  the  same  size)  made  of  leather  ;  but  the  cannon-ball  will 
overcome  this  resistance  more  easily^  and  fall  to  the  ground, 
consequently,  quicker  than  the  leather  ball. 

Caroline.  This  is  very  clear,  and  solves  the  difficulty 
perfectly.  The  air  offers  the  same  resistance  to  a  bit  of  lead 
and  a  bit  of  feather  of  the  same  size  ;  yet  the  one  seems  to 
meet  with  no  obstruction  in  its  fall,  whilst  the  other  is 
evidently  resisted  and  supported  for  some  time  by  the  air. 

Emily,  The  larger  the  surface  of  a  body,  then,  the  more 
air  it  covers,  and  the  greater  is  the  resistance  it  meets  with 
from  it. 

Mrs.  B.  Certainly  :  observe  the  manner  in  which  this 
sheet  of  paper  falls  ;  it  floats  awhile  in  the  air,  and  then 
gently  descends  to  the  ground.  I  will  roll  the  same  piece  of 
paper  up  into  a  ball  :  it  offers  now  but  a  small  surface  to  the 
air,  and  encounters  therefore  but  little  resistance :  see  how 
much  more  rapidly  it  falls. 

The  heaviest  bodies  may  be  made  to  float  awhile  in  the  air, 
by  making  the  extent  of  their  surface  counterbalance  their 
weight.  Here  is  some  gold,  which  is  the  most  dense  body 
we  are  acquainted  with,  but  it  has  been  beaten  into  a  very 
thin  leaf,  and  offers  so  great  an  extent  of  surface  in  proportion 
to  its  weight,  that  its  fall,  you  see,  is  still  more  retarded  by 
the  resistance  of  the  air  than  that  of  the  sheet  of  paper. 

Caroline.  That  is  very  curious  ;  and  it  is,  I  suppose,  upon 
the  same  principle  that  iron  boats  may  be  made  to  float  on 
water  ? 

But,  Mrs.  B.,  if  the  air  is  a  real  body,  is  it  not  also  subjec- 
ted to  the  laws  of  gravity  ? 

Mrs.  B.     Undoubtedly: 

Caroline.  Then  v/hy  does  it  not,  like  all  other  bodies, 
fall  to  the  ground  ? 

Mi^s.  B.  On  account  of  its  spring  or  elasticity.  The  air 
is  an  elastic  fluid  ;  a  species  of  bodies,  the  peculiar  property 
of  which  is  to  resume,  after  compression,  their  original  dimen- 
sions ;  and  you  must  consider  the  air  of  which  the  atmosphere 
is  composed  as  existing  in  a  state  of  compression,  for  its  par- 
ticles being  drawn  towards  the  earth  by  gravity,  are  brought 
closer  together  than  they  would  otherwise  be,  but  the  spring 
or  elasticity  of  the  air  by  which  it  endeavours  to  resist  com- 
pression gives  it  a  constant  tendency  to  expand  itself,  so  as 
ro  resimi^^  the  dimensions  it  would  naturally  have,  if  not 


34  ON  THE  ATTRACTION  OF  GRAVITY. 

under  the  influence  of  gravity.  The  air  may  therefore  be  said 
constantly  to  struggle  with  the  power  of  gravity  without 
being  abie  to  overcome  it.  Gravity  thus  confines  the  air  to 
the  regions  of  our  globe,  w^hilst  its  elasticity  prevents  it  from, 
falling  like  other  bodies  to  the  ground. 

Emily,  The  air  then  is  I  suppose,  thicker,  or  I  should 
rather  say  more  dense,  near  the  surface  of  the  earth,  than  in 
the  higher  regions  of  the  atmosphere  ;  for  that  jiart  of  the  air 
which  is  nearer  the  surface  of  the  earth  must  be  most  strongly 
attracted. 

Mrs,  B,  The  diminution  of  the  force  of  gravity,  at  so 
small  a  distance  as  that  to  which  the  atmosphere  extends 
(compared  with  the  size  of  the  earth)  is  so  inconsiderable  as 
to  be  scarcely  sensible  ;  but  the  pressure  of  the  upper  parts 
of  the  atmosphere  on  those  beneath,  renders  the  air  near  the 
surface  of  the  earth  much  more  dense  than  the  upper  regions. 
The  pressm'e  of  the  atmosphere  has  been  compared  to  that  of 
a  pile  of  fleeces  of  wool,  in  which  the  lower  fleeces  are  pressed 
together  by  the  weight  of  those  above  ;  these  lie  light  and 
loose,  in  proportion  as  they  approach  the  uppermost  fleece, 
which  receives  no  external  pressure,  and  is  confined  merely 
by  the  force  of  its  ow  n  gravity. 

Caroline,  It  has  just  occurred  to  me  that  there  are  some 
bodies  w^hich  do  not  gravita,te  tov/a^'ds  the  earth.  Smoke  and. 
steam,  for  instance,  rise  instead  of  falling. 

Mrs,  B,  It  is  still  gravity  which  produces  their  ascent ; 
at  least,  were  that  power  destroyed,  these  bodies  would  not  rise. 

Caroline,  I  shall  be  out  of  conceit  with  gravity,  if  it  is  so 
inconsistent  in  its  operations. 

Mrs.  B.  There  is  no  difficulty  in  reconciling  this  apparent 
inconsistency  of  eflect.  The  air  near  the  earth  is  heavier 
than  smoke,  steam  or  other  vapours  ;  it  consequently  not 
only  suppoits  these  light  bodies,  but  forces  them  to  rise,  till 
they  reach  a  part  of  the  atmosphere,  the  weight  of  w^hich  is 
not  greater  than  their  own,  and  then  they  remain  stationary. 
Look  at  this  basin  of  water  :  why  does  the  piece  of  paper 
which  I  throw  into  it  float  on  the  surface  ? 

Emihj,  Because,  being  lighter  than  the  water,  it  is  sup- 
ported by  it. 

Mrs,  B.  And  now  that  I  pour  more  water  into  the  basin, 
why  does  the  paper  rise  ? 

Emihj,  The  water  being  heavier  tlian  the  paper,  gets 
beaeatli  it,  and  obliges  it  to  rise. 


0>»  THE  ATTRACTION  OF  GRAVITY.  Sb 

Mrs.  B.  In  a  similar  manner  are  smoke  and  vapour  forced 
upwards  by  the  air  ;  but  these  bodies  do  not,  hke  the  paper, 
ascend  to  the  surface  of  the  fluid,  because,  as  we  observed 
before,  the  air  being  thinner  and  lighter  as  it  is  more  distant 
from  the  earth,  vapours  rise  only  till  they  attain  a  region  of 
air  of  their  own  density.  Smoke,  indeed,  ascends  but  a  very 
little  way  ;  it  consists  of  minute  particles  of  fuel  carried  up 
by  a  current  of  heated  air  from  the  fire  below  :  heat,  you 
recollect,  expands  all  bodies ;  it  consequently  rarefies  air,  and 
renders  it  lighter  than  the  colder  air  of  the  atmosphere  ;  the 
heated  air  from  the  fire  carries  up  with  it  vapour  and  small 
particles  of  the  combustible  materials  which  are  burning  in 
the  fire.  When  ih.s  current  of  hot  air  is  cooled  by  mixing 
with  that  of  the  atmosphere,  the  minute  particles  of  coal  or 
other  combustible  fall,  and  it  is  this  which  produces  the  small 
black  flakes  which  render  the  air  and  every  thing  in  contact 
with  it,  in  London,  so  dirty. 

Caroline.  You  must,  however,  allow  me  to  make  one 
more  objection  to  the  universal  gravity  of  bodies  ;  which  is 
the  ascent  of  air  balloons,  the  materials  of  which  are  un- 
doubtedly heavier  than  air  ;  how,  therefore,  can  they  be 
supported  by  it  ? 

Mrs.B.  I  admit  that  the  materials  of  which  balloons  are 
made  are  heavier  than  the  air  ;  but  the  air  with  which  they 
are  filled  is  an  elastic  fluid,  of  a  different  nature  from  the 
atmospheric  air,  and  considerably  lighter ;  so  that  on  the 
whole,  the  balloon  is  lighter  than  the  air  which  it  displaces, 
and  consequently  will  rise,  on  the  same  principle  as  smoke 
and  vapour.  Now,  Emily,  let  me  hear  if  you  can  explain 
how  the  gravity  of  bodies  is  modified  by  the  effect  of  the  air  ? 

Emily.  The  air  forces  bodies  which  are  lighter  than  itself 
to  ascend  ;  those  that  are  of  an  equal  weight  will  remain 
stationary  in  it ;  and  those  that  are  heavier  will  descend 
through  it  :  but  the  air  will  have  some  effect  on  these  last ; 
for  if  they  are  not  much  heavier,  they  will  with  difficulty 
overcome  the  resistance  they  meet  with  in  passing  through  it, 
they  will  be  borne  up  by  it,  and  their  fall  will  be  more  or  less 
retarded. 

Mrs.  B.  Very  well.  Observe  how  slowly  this  light 
feather  falls  to  the  ground,  while  a  heavier  body,  like  this 
marble,  overcomes  the  resistance  which  the  air  makes  to  its 
descent  much  more  easily,  and  its  fall  is  proportionally  more 
rapid.     I  now  throw  a  pebble  into  this  tub  of  water  ;  it  does 


S6  ON  THE  ATTRACTION  OF  GRAVITV. 

not  reach  the  bottom  near  so  soon  as  if  there  were  no  water  in 
the  tub,  because  it  meets  with  resistance  from  the  water. 
Suppose  that  we  could  empty  the  tub,  not  only  of  water,  but 
of  air  also,  the  pebble  would  then  fall  quicker  still,  as  it  would 
in  that  case  meet  with  no  resistance  at  all  to  counteract  its 
gravity. 

Thus  you  see  that  it  is  not  the  different  degrees  of  gravity, 
but  the  resistance  of  the  air,  which  prevents  bodies  of  different 
weight  from  falling  with  equal  velocities  ;  if  the  air  did  not 
bear  up  the  feather,  it  would  reach  the  ground  as  soon  as  the 
marble. 

Ccu'oline,  I  make  no  doubt  that  it  is  so  ;  and  yet  I  do  not 
feel  quite  satisfied.  I  wish  there  was  any  place  void  of  air, 
in  which  the  experiment  could  be  made. 

Mrs,  B.  If  that  proof  will  satisfy  your  doubts,  I  can  give 
it  you.  Here  is  a  machine  called  an  air pump^{^g,  2.  pi.  I.) 
by  means  of  which  the  air  may  be  expelled  from  any  close 
vessel  which  is  placed  over  this  opening,  through  which  the 
air  is  pumped  out.  Glasses  of  various  shapes,  usually  called 
receivers,  are  employed  for  this  purpose.  We  shall  now 
exhaust  the  air  from  this  tall  receiver  which  is  placed  over  the 
opening,  and  we  shall  find  that  bodies  of  whatever  weight  or 
size  within  it,  will  fall  from  the  top  to  the  bottom  in  the  same 
space  of  time. 

Caroline.  Oh,  I  shall  be  delighted  with  this  experiment  ; 
what  a  curious  machine  !  how  can  you  put  the  two  bodies  of 
different  weight  within  the  glass,  without  admitting  the  air. 

Mrs,  B,  A  guinea  and  a  feather  are  already  placed  there 
for  the  purpose  of  the  experiment  :  here  is,  you  see,  a  contri- 
vance to  fasten  them  in  the  upper  part  of  the  glass  ;  as  soon 
as  the  air  is  pumped  out,  I  shall  turn  this  little  screw,  by 
which  means  the  brass  plates  which  support  them  will  be 
inclined,  and  the  two  bodies  will  fall. — Now  I  believe  I  have 
pretty  well  exhausted  the  air. 

Caroline,  Pray  let  me  turn  the  screw.  I  declare,  they 
both  reached  the  bottom  at  the  same  instant  !  Did  you  see, 
Emily,  the  feather  appeared. as  heavy  as  the  guinea  ? 

Emily,  Exactly  ;  and  fell  just  as  quickly.  How  won- 
derful this  is  !  what  a  number  of  entertaining  experiments 
might  be  made  with  this  machine  ! 

Mrs,  B,  No  doubt  there  are  a  great  variety ;  but  we  shall 
reserve  them  to  elucidate  the  subjects  to  which  they  relate  : 
if  I  had  not  explained  to  you  why  the  guinea  and  the  feather 


ON  THE  ATTRACTION  OF  GRAVITY.  37 

ieil  with  equal  velocity,  you  would  not  have  been  so  well 
pleased  with  the  experiment. 

Emily,  I  should  have  been  as  much  surprised,  but  not  so 
much  interested  ;  besides,  experiments  help  to  imprint  on  the 
memory  the  facts  they  are  intended  to  illustrate  ;  it  will  be 
better  therefore  for  us  to  restrain  our  curiosity,  and  wait  for 
other  experiments  in  their  proper  places. 

Caroline,  Pray  by  what  means  is  the  air  exhausted  in  this 
receiver. 

Mrs.  B,  You  must  learn  something  of  mechanics  in  order 
to  understand  the  construction  of  a  pump.  At  our  next  meet- 
ing, therefore,  I  shall  endeavour  to  make  you  acquainted  with 
the  laws  of  motion,  as  an  introduction  to  that  subject. 


CONVERSATION  III. 


ON  THE  LAWS  OF  MOTION. 

On  Motion  ;  Of  the  Inertia  of  Bodies  ;  Of  Force  to  Produce 
Motion  ;  Direction  of  Motion  ;  Velocity,  Absolute  and 
Relative  ;  Uniform  Motion  ;  Retarded  Motion  ;  Accele- 
rated Motion  ;  Velocity  of  Falling  Bodies  ;  Momentum  ; 
Action  and  Re-action  Equal ;  Elasticity  of  Bodies  ; 
Porosity  of  Bodies  ;  Refected  Motion  y-  Angles  of  Inci- 
dence and  Reflection, 


MRS.  B. 

The  science  of  mechanics  is  founded  on  the  laws  of  motion  ; 
it  will,  therefore,  be  necessary  to  make  you  acquainted  with 
these  laws  before  we  examine  the  mechanical  powers.  Tell 
me,  Caroline,  what  do  you  understand  by  the  word  motion  ? 

Qftroline.  1  think  I  understand  it  perfectly,  though  I  am 
at  a  loss  to  describe  it.  Motion  is  the  act  of  moving  about, 
going  from  one  place  to  another,  it  is  the  contrary  of  remain- 
ing at  rest. 

Mrs,  B.  Very  well.  Motion  then  consists  in  a  change  of 
place  ;  a  body  is  in  motion  whenever  it  is  changing  its  situa- 
tion with  regard  to  a  fixed  point. 

Now  since  we  have  observed  that  one  of  the  general  pro- 
perties of  bodies  is  Inertia,  that  is,  an  entire  passiveness  either 
with  regard  to  motion  or  rest,  it  follows  that  a  body  cannot 
move  without  being  put  into  motion  ;  the  power  which  puts  a 
body  into  motion  is  called  force ;  thus  the  stroke  of  the 
hammer  is  the  force  which  drives  the  nail  ;  the  pulling  of  the 
horse  that  which  draws  the  carriage,  &c.  Force  then  is  the 
cause  which  produces  motion. 

Emily,  And  may  we  not  say  that  gravity  is  the  force 
which  occasions  the  fall  of  bodies  ? 


ON  THE    LAWS  OP  MOTION.  S\) 

Mrs.  B.  Undoubtedly.  I  had  given  you  the  most  familiar 
illustrations  in  order  to  render  the  explanation  clear  ;  but 
since  you  seek  for  more  scientific  examples,  you  may  say  that 
cohesion  is  the  force  which  binds  the  particles  of  bodies  to- 
gether, and  heat  that  which  drives  them  asunder. 

The  motion  of  a  body  acted  upon  by  a  single  force  is  always 
in  a  straight  line,  in  the  direction  in  which  it  received  the 
impulse. 

Caroline.  That  is  very  natural  ;  for  as  the  body  is  inert, 
and  can  move  only  because  it  is  impelled,  it  will  move  only 
in  the  direction  in  wiiich  it  is  impelled.  The  degree  of  quick- 
ness wath  which  it  moves,  must,  I  suppose,  also  depend  upon 
the  degree  of  force  with  which  it  is  impelled. 

Mrs.  B.  Yes  ;  the  rate  at  which  a  body  moves,  or  the 
shortness  of  the  time  which  it  takes  to  move  from  one  place 
to  another,  is  called  its  velocity  ;  and  it  is  one  of  the  laws  of 
motion  that  the  velocity  of  the  moving  body  is  proportional 
to  the  force  by  which  it  is  put  in  motion.  We  must  distin- 
guish between  absolute  and  relative  velocity. 

The  velocity  of  a  body  is  called  absolute^  if  we  consider 
the  motion  of  the  body  in  space,  without  any  reference  to  that 
of  other  bodies.  When  for  instance  a  horse  goes  fifty  miles 
in  ten  hours,  his  velocity  is  five  miles  an  hour. 

The  velocity  of  a  body  is  termed  relative^  when  compared 
with  that  of  another  body  which  is  itself  in  motion.  For 
instance,  if  one  man  w^alks  at  the  rate  of  a  nnle  an  hour,  and 
another  at  the  rate  of  two  miles  an  hour,  the  relative  velocity 
of  the  latter  is  double  that  of  the  former,  but  the  absolute 
velocity  of  the  one  is  one  mile,  and  that  of  the  other  two  miles 
an  hour. 

Emihj.  Let  me  see  if  I  understand  it.  The  relative  ve- 
locity of  a  body  is  the  degree  of  rapidity  of  its  motion  compared 
\Y\t\i  that  of  another  body  ;  thus  if  one  ship  sail  three  times  as 
far  as  another  ship  in  the  same  space  of  time,  the  velocity  of 
the  former  is  equal  to  three  times  that  of  the  latter. 

Mrs.  B.  The  general  rule  may  be  expressed  thus  :  the 
velocity  of  a  body  is  measured  by  the  space  over  w  hich  it 
moves,  divided  by  the  time  which  it  employs  in  that  motion  : 
thus  if  you  travel  one  hundred  miles  in  twenty  hours,  what  is 
your  velocity  in  each  hour  ? 

Emlbf.  I  must  divide  the  space,  which  is  one  hundred 
miles,  by  the  time,  which  is  twenty  hours,  and  the  answer  w  ill 
be  five  miles  an  hour.     Then,  Mrs.  B.,  mav  we  not  reverse 


40  ON  THE  LAWS  OF  MOTION. 

this  mle  and  say,  that  the  time  is  equal  to  the  space  divided  by 
the  velocity  ;  since  the  space  one  hundred  miles,  divided  by 
the  velocity  five  miles,  gives  twenty  hours  for  the  time  ? 

Mrs.  B,  Certainly  ;  and  we  may  say  also  that  space  is 
equal  to  the  velocity  multiplied  by  the  time.  Can  you  tell 
me,  Caroline,  how  many  miles  you  will  have  travelled,  if  your 
velocity  is  three  miles  an  hour  and  3  ou  travel  six  hours  ? 

Caroline,  Eighteen  miles  ;  for  the  product  of  3  multiplied 
by  6,  is  18. 

Mrs.  B.  I  suppose  that  you  understand  what  is  meant  by 
the  terms  uniform^  accelerated  and  retarded  motion. 

Emili/.  I  conceive  uniform  motion  to  be  that  of  a  body 
whose  motion  is  regular,  and  at  an  equal  rate  throughout  ; 
for  instance,  a  horse  that  goes  an  equal  number  of  miles  every 
hour.  But  the  hand  of  a  w^atch  is  a  much  better  example,  as 
its  motion  is  so  regular  as  to  indicate  the  time. 

M7^s.  B.  You  have  a  right  idea  of  uniform  motion  ;  but  it 
would  be  more  correctly  expressed  by  saying,  that  the  motion 
of  a  body  is  uniform  when  it  passes  over  equal  spaces  in  equal 
times.  Uniform  motion  is  produced  by  a  force  having  acted 
on  a  body  once,  and  having  ceased  to  act  ;  as  for  instance^ 
the  stroke  of  a  bat  on  a  cricket  ball. 

Caroline.  But  the  motion  of  a  cricket  ball  is  not  uniform  ; 
its  velocity  gradually  diminishes  till  it  falls  to  the  ground. 

Mrs.  B.  Recollect  that  the  cricket  ball  is  inert,  and  has 
no  more  power  to  stop  than  to  put  itself  in  motion  ;  if  it  falls, 
therefore,  it  must  be  stopped  by  some  force  superior  to  that 
hy  wlTich  it  was  projected,  and  v/hich  destroys  its  motion. 

Caroline.  And  it  is  no  doubt  the  force  of  gravity  which 
3ui:teracts  and  destroys  that  of  projection  ;  but  if  there  were 

)  such  power  as  gravity,  would  the  cricket  ball  never  stop  ? 

Mrs.  B.  If  neither  gi-avity  nor  any  .other  force,  such  a:> 
ilie  resistance  of  the  air,  opposed  its  motion,  the  cricket  ball, 
or  even  a  stone  thrown  by  the  hand,  would  proceed  onwards 
in  a  right  line,  and  with  an  uniform  velocity  for  ever. 

Caroline.  You  astonish  me  !  I  thought  that  it  was  impos- 
sible to  produce  perpetual  motion  ? 

Mrs.  B.  Perpetual  motion  cannot  be  produced  by  art, 
because  gravity  ultimately  destroys  all  motion  that  human 
powers  can  produce. 

Emily.  But  independently  of  tlie  force  of  gravity,  I  can- 
not conceive  that  the  little  motion  I  am  capable  of  giving  to 
a  stone  would  put  it  in  motion  for  ev^r. 


ON  THE  LAWS  OF  MOTIOIS.  41 

Mrs,  B,  The  quantity  of  motion  you  communicate  to  the 
stone  would  not  influence  its  duration  ;  if  you  threw  it  with 
httle  force  it  would  move  slowly,  for  its  velocity,  you  must 
remember,  will  be  proportional  to  the  force  with  which  it  is 
projected  ;  but  if  there  is  nothing  to  obstruct  its  passage,  it 
will  continue  to  move  with  the  same  velocity,  and  in  the  same 
direction  as  when  you  first  projected  it. 

Caroline.  This  appears  to  me  quite  incomprehensible  ; 
we  do  not  meet  with  a  single  instance  of  it  in  nature. 

Mrs,  B,  I  beg  your  pardon.  When  you  come  to  study 
the  motion  of  the  celestial  bodies,  you  will  find  that  nature 
abounds  with  examples  of  perpetual  motion  ;  and  that  it 
conduces  as  much  to  the  harmony  of  the  system  of  the  universe, 
as  the  prevalence  of  it  would  to  the  destruction  of  all  comfort 
on  our  globe.  The  wisdom  of  Providence  has  therefore 
ordained  insurmountable  obstacles  to  perpetual  motion  here 
below,  and  though  these  obstacles  often  compel  us  to  contend 
with  great  difficulties,  yet  there  results  from  it  that  order, 
regularity  and  repose,  so  essential  to  the  preservation  of  all 
the  various  beings  of  which  this  world  is  composed. 

Now  can  you  tell  me  what  is  retarded  motion  ? 

Caroline,  Retarded  motion  is  that  of  a  body  which  moves 
every  moment  slower  and  slower  :  thus  when  I  am  tired  with 
walking  fast,  I  slacken  my  pace  ;  or  when  a  stone  is  thrown 
upwards,  its  velocity  is  gradually  diminished  by  the  power  of 
gravity. 

Mrs,  B,  Retarded  motion  is  produced  by  some  force  act- 
ing upon  the  body  in  a  direction  opposite  to  that  which  first 
put  it  in  motion  :  you  who  are  an  animated  being,  endowed 
with  power  and  will,  may  slacken  your  pace,  or  stop  to  rest 
when  you  are  tired  ;  but  inert  matter  is  incapable  of  any  feel- 
ing of  fatigue,  can  never  slacken  its  pace,  and  never  stop, 
unless  retarded  or  arrested  in  its  course  by  some  opposing 
force  ;  and  as  it  is  the  laws  of  inert  bodies  which  mechanics 
treats  of,  I  prefer  your  illustration  of  the  stone  retarded  in  its 
ascent.  Now,  Emily,  it  is  your  turn ;  what  is  accelerated 
motion  ? 

Emili/,  Accelerated  motion,  I  suppose,  takes  place  when 
the  velocity  of  a  body  is  increased  ;  if  you  had  not  objected 
to  our  giving  such  active  bodies  as  ourselves  as  examples,  I 
should  say  that  my  motion  is  accelerated  if  I  change  my  pace 
from  walking  to  running.  I  cannot  think  of  any  instance  of 
4* 


42  ON  THE  LAWS  OF  MOTION. 

accelerated  motion  in  inanimate  bodies  ;  all  motion  of  inert 
matter  seems  to  be  retarded  by  gravity. 

Mrs,  B.  Not  in  all  cases  ;  for  the  power  of  gravitation 
sometimes  produces  accelerated  motion ;  for  instance,  a  stone 
falling  from  a  height  moves  with  a  regularly  accelerated  motion. 

Emihj.  True  ;  because  the  nearer  it  approaches  the  earth, 
the  more  it  is  attracted  by  it. 

Mrs,  B,  You  have  mistaken  the  cause  of  its  acceleration 
of  motion  ;  for  though  it  is  true  that  the  force  of  gravity 
increases  as  a  body  approaches  the  earth,  the  difference  is  so 
trifling  at  any  small  distance  from  its  surface  as  not  to  be 
perceptible. 

Accelerated  motion  is  produced  when  the  force  which  put 
a  body  in  motion  continues  to  act  upon  it  during  its  motion, 
so  that  its  motion  is  continually  increased.  When  a  stone 
falls  from  a  lieight,  the  impulse  which  it  receives  from  gravity 
during  the  first  instant  of  its  fall,  would  be  sufficient  to  bring  it 
to  the  ground  with  a  uniform  velocity  :  for,  as  we  have  ob- 
served, a  body  having  been  once  acted  upon  by  a  force,  will 
continue  to  move  w^itli  a  uniform  velocity  ;  but  the  stone  is 
not  acted  upon  by  gravity  merely  at  the  first  instant  of  its  fall, 
this  power  continues  to  impel  it  during  the  whole  of  its  descent, 
and  it  is  this  continued  impulse  which  accelerates  its  motion. 

Emily,     I  do  not  quite  understand  that. 

Mrs,  B.  Let  us  suppose  that  the  instant  after  you  have  let 
fall  a  stone  from  a  high  tower,  the  force  of  gravity  were  anni- 
liilated,  the  body  would  nevertheless  continue  to  move  down- 
wards, for  it  v/ould  have  received  a  first  impulse  from  gravity, 
and  a  body  once  put  in  motion  will  not  stop  unless  it  meets 
with  some  obstacle  to  impede  its  course  ;  in  this  case  its 
velocity  would  be  uniform,  for  though  there  w^ould  be  no 
obstacle  to  obstruct  its  descent,  there  would  be  no  force  to 
accelerate  it. 

Emily,     That  is  very  clear. 

Mrs,  B,  Then  you  have  only  to  add  the  power  of  gravity 
constantly  acting  on  the  stone  during  its  descent,  and  it  will 
not  be  difficult  to  understand  that  its  motion  will  become 
accelerated,  since  the  gravity  which  acts  on  the  stone  during 
the  first  instant  of  its  descent,  will  continue  in  force  every 
instant  till  it  reaches  the  ground.  Let  us  suppose  that  the 
impulse  given  by  gravity  to  the  stone  during  the  first  instant 
of  its  descent  be  equal  to  one^  the  next  instant  we  shall  find 


ON  THE  LAWS  OF  MOTION.  46 

that  an  additional  impulse  gives  the  stone  an  additional  velo- 
city equal  to  one,  so  that  the  accumulated  velocity  is  now 
equal  to  two  ;  the  following  instant  another  im.pulse  increases 
the  velocity  to  three,  and  so  on  till  the  stone  reaches  the 
ground. 

Caroline.  Now  I  understand  it ;  the  effects  of  preceding 
impulses  must  be  added  to  the  subsequent  velocities. 

Blrs,  B,  Yes  ;  it  has  been  ascertained  both  by  experiment 
and  calculations,  which  it  would  be  too  difficult  for  us  to  enter 
into,  that  heavj/  bodies  descending  from  a  height  by  the  force 
of  gravity,  fall  sixteen  feet  the  first  second  of  time,  three  times 
that  distance  in  the  next,  five  times  in  the  third  second,  seven 
times  in  the  fourth,  and  so  on,  regularly  increasing  their  velo- 
cities according  to  the  number  of  seconds  during  which  tlie 
body  has  been  falling. 

Emily.  If  you  throw  a  stone  perpendicularly  upwards,  is 
it  not  the  same  length  of  time  ascending  that  it  is  descending. 

Mrs.  B.  Exactly ;  in  ascending,  the  velocity  is  diminished 
by  the  force  of  gravity  ;  in  descending,  it  is  accelerated  by  it. 

Caroline.  I  should  then  have  imagined  that  it  would  have 
fallen  quicker  than  it  rose  ? 

Mrs.  B.  You  must  recollect  that  the  force  with  v>"hich  it  is 
projected  must  be  taken  into  the  account ;  and  that  this  force 
is  overcome  and  destroyed  by  gravity  before  the  body  falls. 

Caroline.  But  the  force  of  projection  given  to  a  stone  in 
throwing  it -upwards,  cannot  alwajs  be  equal  to  the  force  of 
gravity  in  bringing  it  down  again,  for  the  force  of  gravity  is 
always  the  same,  whilst  the  degree  of  impulse  given  to  the 
stone  is  optional ;  I  may  throw  it  up  gently  or  v/itli  violence. 

Mrs.  B.  If  you  throw  it  gently,  it  will  not  rise  high  ;  per- 
haps only  sixteen  feet,  in  which  case  it  will  fall  in  one  second 
of  tiine.  Now  it  is  proved  by  experiment,  tha,t  an  impulse 
requisite  to  project  a  body  sixteen  feet  upwards,  will  make  it 
ascend  that  height  in  one  second  ;  here  then  the  times  of  the 
ascent  and  descent  are  equal.  But  supposing  it  be  required 
to  throw  a  stone  twice  that  height,  the  force  must  •be  propor- 
tionally greater. 

You  see  then,  that  the  impulse  of  projection  in  throwing  a 
body  upwards,  is  always  equal  to  the  action  of  the  force  ot 
gravity  during  its  descent ;  and  that  it  is  the  greater  or  less 
distance  to  which  the  body  rises,  that  makes  these  two  forces 
balance  each  other. 

I  must  now  explain  to  you  what  is  meant  by  the  momeninm 


44  ON  THE  LAWS  OF  MOTION. 

of  bodies.  It  is  the  force,  or  power,  with  which  a  body  in 
motion,  strikes  against  another  body.  The  momentum  of  a 
bod}^  is  composed  of  its  quantity  of  matter^  muhiphed  by  its 
quantity  of  motion  ;  in  other  words,  its  weight  and  its  velocity. 

Caroline,  The  quicker  a  body  moves,  the  greater,  no 
doubt,  must  be  the  force  with  which  it  would  strike  aeainst 
another  body. 

Emily.  Therefore  a  small  body  may  have  a  greater  mo- 
mentum than  a  large  one,  provided  its  velocity  be  sufficiently 
greater  ;  for  instance,  the  momentum  ol'  an  arrow  shot  from 
a  bow,  must  be  greater  than  a  stone  throvv^n  by  the  hand. 

Caroline,  We  know  also  by  experience,  that  the  heavier 
a  body  is,  the  greater  is  its  force  ;  it  is  not  therefore  difficult 
to  understand,  that  the  whole  power  or  momentum  of  a  body 
must  be  composed  of  these  two  properties  :  but  I  do  not  under- 
stand, why  they  should  be  muUipUed^  the  one  by  the  other  ; 
I  should  have  supposed  fTiat  the  quantity  of  matter  should 
have  been  added  to  the  quantity  of  motion  ? 

Mrs,  B,  It  is  found  by  experiment,  tliat  if  the  weight  of  a 
body  is  represented  by  the  number  3,  and  its  velocity  also  by 
3,  its  momentum  will  be  represented  by  9  ;  not  6,  as  would 
be  the  case,  were  these  figures  added,  instead  of  being  multi- 
plied together.  I  recommend  it  to  you  to  be  careful  to  remem- 
ber the  definition  of  the  momentum  of  bodies,  as  it  is  one  of 
the  most  important  points  in  mechanics  ;  you  will  find,  that 
it  is  from  opposing  motion  to  matter,  that  machines  derive 
their  powers.* 

The  re-action  of  bodies,  is  the  next  law  of  motion  which  I 
must  explain  to  you.  When  a  body  in  motion  strikes  against 
another  body,  it  meets  with  resistance  from  it ;  the  resistance 
of  the  bod}^  at  rest,  will  be  equal  to  the  blow  struck  by  the 
body  in  motion ;  or  to  express  myself  in  philosophical  language,. 
action  and  re-action  will  be  equal,  and  in  opposite  directions. 

Caroline,  Do  you  mean  to  say,  that  the  action  of  the 
body  which  strikes,  is  returned  with  equal  force  by  the  body 
which  receives  the  blow, 

*  Tn  comparing  together  the  momenta  of  different  bodies,  we  must  be 
attentive  to  measure  their  Meighis  and  velocities,  by  the  same  denomina- 
tion of  weights  and  of  spaces,  otherwise  the  results  woald  not  agree. 
Thus  if  we  estimate  tlie  weight  of  OJie  body  in  on?  ces,  we  must  estimate 
the  weight  of  the  rest  also  in  ounces,  and  not  in  pounds  ;  and  in  compu- 
ti'igthe  velocities,  in  Tike  manner,  we  should  adhere  to  the  same  standai*(l 
of  measure,  both  of  space  and  of  time;  as  for  instance,  the  number  of 
feet  in  one  second,  or  of  miles  in  one  hour. 


ON  THE  LAWS  OF  MOTION.  45 

Mrs,  B.     Exactly. 

Caroline.  But  if  a  man  strikes  another  on  the  face  with 
his  fistj  he  surely  does  not  receive  as  much  pain  by  the  re- 
action as  he  inflicts  by  the  blow  ? 

Mrs.  B.  iVo  ;  but  this  is  simply  owing  to  the  knuckles 
having  much  less  feeling  than  the  face. 

Here  are  two  ivory  balls  suspended  by  direads,  (plate  1. 
fig.  3.)  draw  one  of  them,  A,  a  little  on  one  side, — now  let  it 
go  ; — it  strikes,  you  see,  against  the  other  ball  B,  and  drives 
it  off,  to  a  distance  equal  to  that  through  which  the  first  ball 
fell  ;  but  the  motion  of  A  is  stopped,  because  when  it  struck 
B,  it  received  in  return  a  blov/  equal  to  that  it  gave,  and  its 
motion  w^as  consequently  destroyed. 

Emily.  I  should  have  supposed  that  the  motion  of  the  ball 
A  was  destroyed,  because  it  had  communicated  all  its  motion 
to  B. 

Mrs.  B.  It  is  perfectly  true,  that  when  one  body  strikes 
against  another,  the  quantity  of  motion  communicated  to  the 
second  body,  is  lost  by  the  first ;  but  this  loss  proceeds  from 
the  action  of  the  body  which  is  struck. 

Here  are  six  ivory  balls  hanging  in  a  row,  (fig.  4.)  draw 
the  first  out  of  the  perpendicular,  and  let  it  fall  against  the 
second.  None  of  the  balls  appear  to  move,  you  see,  except 
the  last,  which  flies  off  as  far  as  the  first  ball  fell  ;  can  you 
explain  this  ? 

Caroline.  I  believe  so.  When  the  first  ball  struck  the 
second,  it  received  a  blow  in  return,  w^hich  destroyed  its  mo- 
tion ;  the  second  ball,  though  it  did  not  appear  to  move,  must 
have  struck  against  the  third  ;  the  re-action  of  which  set  it  at 
rest ;  the  action  of  the  third  ball  must  have  been  destroyed 
by  the  re-action  of  the  fourth,  and  so  on  till  motion  was 
commxunicated  to  the  last  ball,  which,  not  being  re-acted  upon, 
ifies  off. 

M/'5.  B.  Very  v/ell  explained.  Objerve,  that  it  is  only 
when  bodies  are  clastic,  as  these  ivory  balls  are,  that  the 
stroke  returned  is  equal  to  the  stroke  given.  I  w^ill  show  you 
!«the  difference  Vv'ith  these  tv/o  balls  of  clay,  (fig.  5.)  which  are 
not  elastic  ;  when  you  raise  one  of  these,  D,  out  of  the  per- 
pendicular, and  let  it  fall  against  the  other,  E,  the  re-action 
of  the  Ia,tter,  on  account  of  its  not  being  elastic,  is  not  sufficient 
to  destroy  the  motion  of  the  former  ;  only  part  of  the  motion 
of  D  will  be  communicated  to  E,  and  the  tw^o  balls  will  move 


16 


ON  THE  LAWS  OF  MOTION. 


on  together  to  d  and  e,  which  is  not  to  so  great  a  distance  as 
that  tlirough  whicli  D  fell. 

Observe  how  useful  re-action  is  in  nature.  Birds  in  flying 
strike  the  air  with  their  wings,  and  it  is  the  re-action  of  the  air 
which  enables  them  to  rise,  or  advance  forwards  ;  re-action 
being  always  in  a  contrary  direction  to  action. 

Caroline,  I  thought  that  birds  might  be  lighter  than  the 
air,  when  their  wings  were  expanded,  and  by  that  means 
enabled  to  fly. 

.'  rs,B.  \Vhen  their  wings  are  spread,  they  are  better 
supported  by  the  air,  as  they  cover  a  greater  extent  of  sur- 
face ;  but  the}'  are  still  much  too  heavy  to  remain  in  that  situ- 
ation, without  continually  flapping  their  wings,  as  you  may 
have  noticed,  w hen  biids  hover  over  their  nests  :  the  force 
with  which  their  wings  strike  against  the  air  must  equal  the 
weight  of  their  bodies,  in  order  that  the  re-action  of  the  air 
may  be  able  to  support  that  weight  ;  the  bird  will  then  re- 
main stationary.  If  the  stroke  of  the  wings,  is  greater  than 
is  required  merely  to  support  the  bird,  the  re-action  of  the  air 
will  make  it  rise  ;  if  it  be  less,  it  will  gently  descend  ;  and 
you  may  have  observed  the  lark,  sometimes  remaining  with 
its  wings  extended,  but  motionless  :  in  this  state  it  drops 
rapidly  into  its  nest. 

Caroline,  What  a  beautiful  effect  this  is  of  the  law  of  re-ac- 
tion !  But  if  flying  is  merely  a  mechanical  operation,  Mrs.  B.^ 
why  should  we  not  construct  wings,  adapted  to  the  size  of  our 
bodies,  fasten  them  to  our  shoulders,  move  them  with  our 
arms,  and  soar  into  the  air. 

Mrs,  B,  Such  an  experiment  has  been  repeatedly  at- 
tempted, but  never  with  success  ;  and  it  is  now  considered 
?.s  totally  impracticable.  The  muscular  power  of  birds  is 
greater  in  proportion  to  their  weight  than  that  of  man  ;  were 
we  therefore  furnished  with  wings  sufficiently  large  to  enable 
us  to  fly,  we  should  not  have  strength  to  put  them  in  motion. 

In  swimming,  a  similar  action  is  produced  on  the  water,  as 
that  on  the  air  in  flying  ;  and  also  in  rov/ing  ;  you  strike  the 
water  with  the  oars,  in  a  direction  opposite  to  that  in  tvhich  the 
boat  is  required  to  move,  and  it  is  the  re-action  of  the  water 
on  the  oars  which  drives  the  boat  along. 

Emily,  You  said,  that  it  was  in  ehistlc  bodies  only,  that 
re-action  was  equal  to  action  ;  pray  what  l^odles  are  elastic 
besides  the  air. 


ON  THE  LAWS  OF  MOTION.  47 

Mrs,  B.  In  speaking  of  the  air,  I  think  we  defined  elasti- 
city to  be  a  property,  by  means  of  which  bodies  that  are 
compressed  returned  to  their  former  state.  If  I  bend  this 
,  cane,  as  soon  as  I  leave  it  at  liberty  it  recovers  its  former 
■  position;  if  I  press  my  finger  upon  your  arm,  as  soon  as  I 
remove  it,  the  flesh,  by  virtue  of  its  elasticity,  rises  and  des- 
troys the  impression  I  made.  Of  all  bodies,  the  air  is  the 
most  eminent  for  this  property,  and  it  has  thence  obtained  the 
name  of  elastic  fluid.  Hard  bodies  are  in  the  next  degree 
elastic  ;  if  two  ivory,  or  metallic  balls  are  struck  together,  the 
parts  at  which  they  touch  will  be  flattened  :  but  their  elasticity 
will  make  them  instantaneously  resume  their  former  shape. 

Caroline,  But  when  two  ivory  balls  strike  against  each 
other,  as  they  constantly  do  on  a  biUiard  table,  no  mark  or 
impression  is  made  by  the  stroke. 

Mrs.  B.  I  beg  your  pardon  ;  but  you  cannot  perceive 
any  mark,  because  their  elasticity  instantly  destroys  all  trace 
of  It. 

Soft  bodies,  which  easily  retain  impression,  such  as  clay, 
wax,  tallow,  butter,  &c.  have  very  little  elasticity  ;  but  of  all 
descriptions  of  bodies  liquids  are  the  least  elastic. 

Emily,  If  sealing-wax  were  elastic,  instead  of  retaining 
the  impression  of  a  seal,  it  would  resume  a  smooth  surface  as 
soon  as  the  weight  of  the  seal  was  removed.  But  pray  what 
is  it  that  produces  the  elasticity  of  bodies  ? 

Mrs,  B.  There  is  great  diversity  of  opinion  upon  that 
point,  and  I  cannot  pretend  to  decide  which  approaches 
nearest  to  the  truth.  Elasticity  implies  susceptibility  of  com- 
pression, and  the  susceptibility  of  compression,  depends  upon 
the  porosity  of  bodies,  for  were  there  no  pores  or  spares 
between  the  particles  of  matter  of  which  a  body  is  composed, 
it  could  not  be  compressed. 

Caroline,  That  is  to  say,  that  if  the  particles  of  bodies 
were  as  close  together  as  possible,  they  cuuld  not  be  squeezed 
closer. 
j^  Emily,  Bodies  then,  whose  particles  are  most  distant 
from  each  other,  must  be  most  susceptible  of  compression,  and 
consequently  most  elastic  ;  and  this  you  say  is  the  case  with 
air,  which  is  perhaps  the  least  dense  of  all  bodies  ? 

Mrs.  B,  You  will  not  in  general  find  this  rule  hold  good, 
for  liquids  have  scarcely  any  elasticity,  whilst  hard  bodies  are 
eminent  for  this  property,  though  the  latter  are  certainly  of 
much  greater  density  than  the  former  ;  elasticity  implies, 


48  uiN  THE  LA^^s  of  motion. 

therefore,  not  only  a  susceptibility  of  compression,  but  depends 
upon  the  power  of  resumincr  its  former  state  after  compression. 

Caroline,  But  surely  there  can  be  no  pores  in  ivory  and 
metals,  Mrs.  B. ;  how  then  can  they  be  susceptible  of 
compression  ? 

Mrs,  B,  The  pores  of  such  bodies  are  invisible  to  the 
naked  eye,  but  you  must  not  thence  conclude  that  they  have 
none  ;  it  is,  on  the  contrary,  well  ascertained  that  gold,  one 
of  the  most  dense  of  all  bodies,  is  extremely  porous,  and  that 
these  pores  are  sufficiently  large  to  admit  water  when  strongly 
compressed  to  pass  through  them.  This  was  shown  by  a 
celebrated  experiment  made  many  years  ago  at  Florence. 

Emily,  If  water  can  pass  through  gold,  there  must  cer- 
tainly be  pores  or  interstices  which  afford  it  a  passage  ;  and  if 
gold  is  so  porous,  what  must  other  bodies  be,  which  are  so 
much  less  dense  than  gold  ! 

Mrs,  B.  The  chief  difference  in  this  respect  is,  I  believe, 
that  the  pores  in  some  bodies  are  larger  than  in  others  ;  in 
cork,  sponge,  and  bread,  they  form  considerable  cavities  ;  in 
wood  and  stone,  when  not  polished,  they  are  generally- 
perceptible  to  the  naked  eye  ;  whilst  in  ivorj-,  metals,  and  all 
varnished  and  polished  bodies,  they  cannot  be  discerned.  To 
give  you  an  idea  of  the  extreme  porosity  of  bodies,  sir  Isaac 
Newton  conjectured  that  if  the  earth  were  so  compressed  as 
to  be  absolutely  without  pores,  its  dimensions  might  possibly 
not  be  more  than  a  cubic  inch. 

Caroline,  What  an  idea  !  Were  we  not  indebted  to  sir 
Isaac  Newton  for  the  theory  of  attraction,  I  should  be  tempted 
to  laugh  at  him  for  such  a  supposition.  What  insignitic^nt 
little  creatures  we  should  be  ! 

Mrs,  B,  If  our  consequence  arose  from  the  size  of  our 
bodies  we  should  indeed  be  but  pigmies,  but  remember  that 
the  mind  of  Newton  was  not  circumscribed  by  the  dimensions 
of  its  envelope. 

Emily,     It  is,  however,  fortunate  that  heat  keeps  the  pores 
of  matter  open  and  distended,  and  prevents  the  attraction  of^ 
cohesion  from  squeezing  us  into  a  nut-shell. 

Mrs.  B.  Let  us  now  return  to  the  subject  of  re-action,  on 
which  we  have  some  further  observations  to  make.  It  is 
re-action,  being  contrary  to  action,  which  produces  reflected 
Qnotion,  If  you  throw  a  ball  against  the  wall,  it  rebounds  ; 
this  return  of  the  ball  is  owing  to  the  re-action  of  the  wall 
against  which  it  struck,  and  is  called  reflected  motion. 


I'i^f.  1. 


PLATE    71. 


ON  THE  LAWS  OF  MOTION.  49 

Emily.  And  I  now  understand  why  balls  filled  with  air 
rebound  better  than  those  stuffed  with  bran  and  wool,  air  being 
most  susceptible  of  compression  and  most  elastic,  the  re-action 
is  more  complete. 

Caroline,  I  have  observed  that  when  I  throw  a  ball 
straight  against  the  wall,  it  returns  straight  to  my  hand  ;  but 
if  I  throw  it  obliquely  upwards,  it  rebounds  still  higher,  and  I 
catch  it  when  it  falls. 

Mrs,  B,  You  should  not  say  straight,  but  perpendicularly 
against  the  wall  ;  for  straight,  is  a  general  term  for  lines  in 
all  directions  which  are  neither  curved  nor  bent,  and  is  there- 
fore equally  applicable  to  oblique  or  perpendicular  lines. 

Caroline,  I  thought  that  perpendicularly  meant  either 
directly  upwards  or  downwards. 

Mrs,  B,  In  those  directions  lines  are  perpendicular  to  the 
earth.  A  perpendicular  line  has  always  a  reference  to  some- 
thing towards  which  it  is  perpendicular  ;  that  is  to  say,  that 
it  inclines  neither  to  the  one  side  nor  the  other,  but  makes  an 
equal  angle  on  every  side.  Do  you  understand  what  an 
angle  is  ? 

Caroline.  Yes,  I  believe  so  :  it  is  two  lines  meeting  in  a 
point. 

Mrs,  B,  Well  then,  let  the  line  A  B  (plate  II,  fig.  1.) 
1'epresent  the  floor  of  the  room,  and  the  line  C  D  that  in  which 
you  throw  a  ball  against  it  ;  the  line  C  D  you  will  observe, 
forms  two  angles  with  the  line  A  B,  and  those  two  angles  are 
equal. 

Emily,  How  can  the  angles  be  equal,  while  the  lines 
which  compose  them  are  of  unequal  length  ? 

Mrs,  B,  An  angle  is  not  measured  by  the  length  of  the 
lines,  but  by  their  opening. 

Emily,  Yet  the  longer  the  lines  are,  the  greater  is  the 
opening  between  them. 

Mrs,  B,  Take  a  pair  of  compasses  and  draw  a  circle  over 
these  angles,  making  the  angular  point  the  centre. 

Emily,     To  what  extent  must  I  open  the  compasses  ? 

Mrs.  K  You  may  draw  the  circle  what  size  you  please, 
provided  that  it  cuts  the  lines  of  the  angles  we  are  to  measure. 
All  circles,  of  whatever  dimensions,  are  supposed  to  be  divided 
into  360  equal  parts,  called  degrees  ;  the  opening  of  an 
angle,  being  therefore  a  portion  of  a  circle,  must  contain  a 
certain  number  of  degrees :  the  larger  the  angle,  the  greater 
5 


UN    I  UK  LAWS  OFMuTlO.N. 


the  number  of  degrees,  and  the  two  angles  are  said  to  be  equal 
when  they  contain  an  equal  number  of  degrees. 

Emily,  Now  I  understand  it.  As  the  dimensions  of  an 
angle  depend  upon  the  number  of  degrees  contained  between 
its  lines,  it  is  the  opening  and  not  the  length  of  its  lines,  which 
determines  the  size  of  the  angle. 

^irs,  B,  Very  well :  now  that  you  have  a  clear  idea  of 
the  dimensions  of  angles,  can  you  tell  me  how  many  degrees 
are  contained  in  the  two  angles  formed  by  one  line  falling 
perpendicular  on  another,  as  in  the  figure  I  have  just  drawn  ? 

Emily,  You  must  allow  me  to  put  one  foot  of  the  com- 
passes at  the  point  of  the  angles,  and  draw  a  circle  round 
them,  and  then  I  think  I  shall  be  able  to  answer  your  ques- 
tion :  the  two  angles  are  together  just  equal  to  half  a  circle, 
they  contain  therefore  90  degrees  each  ;  90  degrees  being  a 
quarter  of  360. 

Mrs,  B,  An  rmgle  of  90  degrees  is  called  a  right  angle, 
and  w^hen  one  line  is  perpendicular  to  another,  it  forms,  you 
see  (fig.  1.)  a  right  angle  on  either  side.  Angles  containing 
more  than  90  degrees  are  called  obtuse  angles  (fig.  2.  ;)  and 
those  containing  less  than  90  degrees  are  called  acute  angles, 
(fig.  3.) 

Caroline,  The  angles  of  this  square  table  are  right  angles, 
but  those  of  the  octagon  table  are  obtuse  angles  ;  and  the  an- 
gles of  sharp-pointed  instruments  are  acute  angles. 

Mrs,  B,  Very  well.  To  return  now  to  your  observation, 
that  if  a  ball  is  thrown  obliquely  against  the  wall  it  will  not 
rebound  in  the  same  direction  ;  tell  me,  have  you  ever  played 
at  billiards  ? 

Caroline,  Yes,  frequently  ;  and  I  have  observed  that 
when  I  push  the  ball  perpendicularly  against  the  cushion  it 
returns  in  the  same  direction  ;  but  when  I  send  it  obliquely 
to  the  cushion,  it  rebounds  obliquely,  but  on  the  opposite 
side ;  the  ball  in  this  latter  case  describes  an  angle,  the  point 
of  which  is  at  the  cushion.  I  have  observed  too,  that  the 
more  obliquely  the  ball  is  struck  against  the  cushion,  the 
more  obliquely  it  rebounds  on  the  opposite  side,  so  that  a 
billiard  player  can  calculate  with  great  accuracy  in  what  di- 
rection it  will  return. 

Mrs,  B,  Very  well.  This  figure  (fig.  4.  plate  II.)  rep- 
resents a  billiard  table  ;  now  if  you  draw  a  line  A  B  from  the 
point  where  the  ball  A  strikes  perpendicular  to  the  cushion  ; 


ox  THE   LAWS  OP  MOTION.  51 

vou  will  find  that  it  will  divide  the  angle  v/hich  the  ball  de- 
scribes into  two  parts,  or  two  angles  ;  the  one  will  show  the 
obliquity  of  the  direction  of  the  ball  in  its  passage  tow^ards 
the  cushion,  the  other  its  obliquity  in  its  passage  back  from 
the  cushion.  The  first  is  called  the  angle  of  incidence^  the 
other  the  angle  of  reflection^  and  these  angles  are  always 
equal. 

Caroline.  This  then  is  the  reason  why,  when  I  throw  a 
ball  obliquely  against  the  wall,  it  rebounds  in  an  opposite 
oblique  direction,  forming  equal  angles  of  incidence  and  of 
reflection. 

Mrs.  B.  Certainly  ;  and  you  will  find  that  the  more  ob- 
liquely you  throw  the  ball,  the  more  obliquely  it  will  rebound. 

We  must  now  conclude  :  but  I  shall  have  some  further 
observations  to  make  upon  the  lav/s  of  motion,  at  our  next 
meeting. 


CONVERSATION  IV. 


ON  COMPOUND  MOTION. 

Compound  Motion^  the  Result  of  two  Opposite  Forces ;  Of 
Circular  Motion^  the  Result  of  tivo  Forces^  one  of  lohich 
confines  the  Body  to  a  Fixed  Point  ;  Centre  of  Motion^ 
the  Point  at  Rest  while  the  other  Parts  of  the  Body  move 
round  it ;  Centre  of  Magnitude,  the  Middle  of  a  Body  ; 
Centripetal  Force ^  that  which  confines  a  Body  to  a  fixed 
Central  Point ;  Centrifugal  Force,  that  which  impels  a 
Body  to  fly  from  the  Centre  ;  Fall  of  Bodies  in  a  Para- 
bola ;  Centre  of  Gravity,  the  Centre  of  Weight,  or  point 
about  which  the  Parts  balance  each  other. 


MRS.  B. 

I  MUST  now  explain  to  you  the  nature  ol'  compound  motion. 
Let  us  suppose  a  body  to  be  struck  by  two  equal  forces  in 
opposite  directions^  how  will  it  move  ? 

Emily,  If  the  directions  of  the  forces  are  in  exact  oppo- 
sition to  each  other,  I  suppose  the  body  would  not  move 
at  all. 

M?'s,  B,  You  are  perfectly  right ;  but  if  the  forces,  instead 
of  acting  on  the  body  in  opposition,  strike  it  in  two  directions 
inclined  to  each  other,  at  an  angle  of  ninety  degrees,  if  the 
ball  A  (fig.  5,  plate  II.)  be  struck  by  equal  forces  at  X  and 
at  Y,  will  it  not  move  ? 

Emily,  The  force  X  would  send  it  towards  B,  and  the 
force  Y  towards  C,  and  since  these  forces  are  equal,  I  do  not 
know  how  the  body  can  obey  one  impulse  rather  than  the 
other,  and  yet  I  think  the  ball  would  move,  because  as  the 
two  forces  do  not  act  in  direct  opposition,  they  cannot  entire- , 
ly  destroy  the  effect  of  each  other 


ON  COMPOUND  MOTION,  53 

Mrs.  B.  Very  true  ;  the  ball  will  therefore  follow  the  di- 
rection of  neither  of  the  forces,  but  will  move  in  a  line  between 
them,  and  will  reach  D  in  the  same  space  of  time,  that  the 
force  X  v/ouldhave  sent  it  to  B,  and  the  force  Y  would  have 
sent  it  to  C.  Now  if  you  drav/  two  lines  from  D,  to  join  B 
and  C,  you  will  form  a  square,  and  the  oblique  line  which  the 
body  describes  is  called  the  diagonal  of  the  square. 

Caroline.  That  is  very  clear,  but  supposing  the  two  forces 
to  be  unequal,  that  the  force  X,  for  instance,  be  twice  as  great 
as  the  force  Y  ? 

Mrs.  B.  Then  the  force  X  would  drive  the  ball  twice  as 
far  as  the  force  Y,  consequently  you  must  draw  the  line  A  B 
{fig.  6.,)  twice  as  long  as  the  line  A  C,  the  body  will  in  this 
case  move  to  D  ;  and  if  you  draw  lines  from  that  point  to  B 
and  C,  you  will  find  that  the  ball  has  moved  in  the  diagonal 
of  a  rectangle. 

Emily.  Allow  me  to  put  another  case  ?  Suppose  the  two 
forces  are  unequal,  but  do  not  act  on  the  ball  in  the  direction 
of  a  right  angle,  but  in  that  of  an  acute  angle,  what  will 
result  ? 

Mrs.  B.  Prolong  the  lines  in  the  directions  of  the  two 
forces,  and  \ou  will  soon  discover  which  way  the  ball  will  be 
impelled  ;  it  will  move  from  A  to  D,  in  the  diagonal  of  a^ 
parallelogram,  (fig.  7.)  Forces  acting  in  the  direction  of  lines 
forming  an  obtuse  angle,  will  also  produce  motion  in  the 
diagonal  of  a  parallelogram.  For  instance,  if  the  body  set  out 
from  B,  instead  of  A,  and  was  impelled  by  the  forces  X  and 
Y,  it  would  move  in  the  dotted  diagonal  B  C. 

We  may  now  proceed  to  circular  motion  :  this  fs  the  result 
of  two  forces  on  a  body,  by  one  of  which  it  is  projected  for- 
v/ard  in  a  right  line,  whilst  by  the  other  it  is  confined  to  a 
fixed  point.  For  instance,  when  I  whirl  this  ball,  which  is 
fastened  to  my  hand  with  a  string,  the  ball  moves  in  a  circular 
direction  ;  because  it  is  acted  on  by  two  forces,  that  which  I 
give  it  which  represents  the  force  of  projection,  and  that  of 
the  string  which  confines  it  to  niy  hand.  If  during  its  motion 
you  were  suddenly  to  cut  the  string,  the  ball  would  fly  off*  in 
a  straight  line  ;  being  released  from  confinement  to  the  fixed 
point,  it  would  be  acted  on  but  by  one  force,  and  motion  pro- 
duced by  one  force,  you  knov/,  is  always  in  a  right  line. 

Caroline.  This  is  a  little  more  difficult  to  comprehend 
than  compound  motion  in  straight  lines. 

Mrs.  B.     You  have  seen  a  mop  trundled,  and  have  observ- 
5* 


54  ON  eo3rpouN»  motion^* 

ed,  that  the  threads  which  compose  the  head  of  the  mop  fly 
from  the  centre  ;  but  being  confined  to  it  at  one  end,  they 
cannot  part  from  it ;  whilst  the  water  they  contain,  being 
unconfined,  is  thrown  off  in  straight  lines. 

Emily.  In  the  same  way,  the  flyers  of  a  windmill,  when 
put  in  motion  by  the  wind,  would  be  driven  straight  forwards 
in  a  right  line,  were  they  not  confined  to  a  fixed  point  round 
which  they  are  compelled  to  move. 

Mrs,  B,  Very  well.  And  observe,  that  the  point  to  which 
the  motion  of  a  small  body,  such  as  the  ball  with  the  stringy 
which  may  be  considered  as  revolving  in  one  plane,  is  confin- 
ed, becomes  the  centre  of  its  motion.  But  when  the  bodies 
are  not  of  a  size  or  shape  to  allow  of  our  considering  every 
part  of  them  as  moving  in  the  same  plane,  they  in  reality  re- 
volve round  a  line,  which  line  is  called  the  axis  of  motion. 
In  a  top,  for  instojice,  when  spinning  on  its  point,  the  axis  is 
the  line  which  passes  through  the  middle  of  it,  perpendicular- 
ly to  the  floor. 

Caroline.  Tlie  axle  of  the  flyers  of  the  windmil?,  is  then 
the  axis  of  its  motion ;  but  is  the  centre  of  motion  always  in 
the  middle  of  a  body  ? 

Mrs.  B.  No,  not  always.  The  middle  point  of  a  body,  is 
called  its  centre  of  magnitude,  or  position,  that  is  the  centre 
of  its  mass  or  bulk.  Bodies  have  also  another  centre,  called 
the  centre  of  gravity,  which  I  shall  explain  to  you  ;  but  at 
present  we  must  confine  ourselves  to  the  axis  of  motion. 
This  line  you  must  observe  remains  at  rest  ;  whilst  all  the 
other  parts  of  the  body  move  around  it ;  when  you  spin  a  top 
the  axis  is  stationary  vrhilst  every  other  part  is  in  motion 
round  it. 

Caroline.  But  a  top  generally  has  a  motion  forwards, 
besides  its  spinning  motion  ;  and  then  no  point  within  it  can 
be  at  rest  ? 

Mrs.  B.  What  I  say  of  the  axis  of  nwtion,  relates  only  to 
circular  motion  ;  that  is  to  say,  to  motion  round  a  line,  and 
not  to  that  which  a  body  may  have  at  the  same  time  in  any 
other  direction.  There  is  one  circumstance  in  circular  mo- 
tion, wliich  you  must  carefully  attend  to  ;  which  is,  that  the 
iiirther  any  part  of  a  body  is  from  the  axis  of  motion,  the 
greater  is  its  velocity  ;  as  you  approach  that  line,  the  velocity 
of  the  parts  gradually  diminish  till  you  reach  the  axis  of 
motion,  which  is  perfectly  at  rest. 

Cwroiine*,    But^  if  every  part  of  the  same  body  did  not 


F^<J•    1. 


PLATE 


Fiq.  6. 


Fia.o- 


^.4.         -   V    I 


%•  7- 


Fi^.  O. 


ON  COMPOUND  MOTION.  55 

move  with  the  same  velocity,  that  part  which  moved  quickest^ 
must  be  separated  from  the  rest  of  the  body,  and  leave  it 
behind  ? 

Mrs.  B.  You  perplex  yourself  by  confounding  the  idea  of 
circular  motion,  with  that  of  motion  in  a  right  line  ;  you  must 
think  only  of  the  motion  of  a  body  round  a  fixed  line,  and  you 
will  find,  that  if  the  parts  farthest  from  the  centre  had  not  the 
greatest  velocity,  those  parts  would  not  be  able  to  keep  up 
with  the  rest  of  the  body,  and  would  be  left  behind.  Do  not 
the  extremities  of  the  vanes  of  a  windmill  move  over  a  much 
greater  space,  than  the  parts  nearest  the  axis  of  motion  ? 
(pi.  III.  fig.  1.)  the  three  dotted  circles  describe  the  paths  in 
which  three  different  parts  of  the  vanes  move,  and  though  the 
circles  are  of  different  dimensions  the  vanes  describe  each  of 
them  in  the  same  space  of  time. 

Carolinp,  Certainly  they  do  ;  and  I  now  only  wonder^ 
that  we  neither  of  us  ever  made  the  observation  before  :  and 
the  same  effect  must  take  place  in  a  solid  body,  like  the  top 
in  spinning  ;  the  most  bulging  part  of  the  surface  must  move 
with  the  greatest  rapidity. 

Mrs,  B.  The  force  which  confines  a  body  to  a  centre, 
round  which  it  moves  is  called  the  centripetal  force  ;  and 
that  force,  which  impels  a  body  to  fly  from  the  centre,  is  cal- 
led the  centrifugal  force  ;  in  circular  motion  these  two  forces, 
constantly  balance  each  other ;  otherwise  the  revolving  body 
would  either  approach  the  centre^  or  recede  from  it,  according 
as  the  one  or  the  other  prevailed. 

Caroline.  When  I  see  any  body  moving  in  a  circle,  I  shall 
remember,  that  it  is  acted  on  by  two  forces. 

Mrs.  B.  Motion,  either  in  a  circle,  an  ellipsis,  or  any 
other  curve-line,  must  be  the  result  of  the  action  of  two  for- 
ces ;  for  you  know,  that  the  impulse  of  one  single  force^  always 
produces  motion  in  a  right  line. 

Emily.  And  if  any  cause  should  destroy  the  centripetal 
force,  the  centrifugal  force  would  alone  impel  the  body,  and 
it  would  I  suppose  fly  off  in  a  s^traight  line  from  the  centre  to 
which  it  had  been  confined. 

Mrs.  B.  It  would  not  fly  off  in  a  right  line  from  the  cen- 
tre ;  but  in  a  right  line  in  the  direction  in  which  it  was  mo- 
ving, at  the  instant  of  its  release  ;  if  a  stone,  whirled-  round 
in  a  sling,  gets  loose  at  the  point  A  (plate  III.  fig,  2.)  it  flies 
off  in  the  direction  A  B  ;  this  line  is  called  a  tangenty  it 
touches  the  circumference  of  the  circlej  and  forms  a  right 


00  ON  COMPOUND  MOTION. 

angle  with  a  line  drawn  from  that  point  of  the  circumference 
to  the  centre  of  the  circle^  C. 

Emily.  You  say,  that  motion  in  a  curve-line,  is  owing  to 
two  forces  acting  upon  a  body  ;  but  when  I  throw  this  ball 
in  a  horizontal  direction^  it  describes  a  curve  line  in  falling  ; 
and  yet  it  is  only  acted  upon  by  the  force  of  projection  ;  there 
is  no  centripetal  force  to  confine  it^  or  produce  compound 
motion. 

Mrs,  B,  A  ball  thus  thrown,  is  acted  upon  by  no  less  than 
three  forces  ;  the  force  of  projection,  which  you  communica- 
ted to  it  ;  the  resistance  of  the  air  thr ough  which  it  passes, 
which  diminishes  its  velocity,  without  changing  its  direction  ; 
and  the  force  of  gravity,  which  finally  brings  it  to  the  ground. 
The  power  of  gravity,  and  the  resistance  of  the  air,  being 
always  greater  than  any  force  of  projection  we  can  give  a 
body,  the  latter  is  gradually  overcome,  and  the  body  brought 
to  the  ground  ;  but  the  stronger  the  projectile  force,  the  lon- 
ger will  these  powers  be  in  subduing  it,  and  the  further  the 
body  will  go  before  it  falls. 

Caroline.  A  shot  fired  from  a  cannon,  for  instance,  will 
go  much  further,  than  a  stone  projected  by  the  hand. 

Mrs.  B.  Bodies  thus  projected,  3^ou  observed,  described  a 
curve-line  in  their  descent  ;  can  you  account  for  that  ? 

Caroline.  No  ;  I  do  not  understand,  why  it  should  not 
fall  in  the  diagonal  of  a  square. 

Mrs.  B.  You  must  consider  that  the  force  of  projection  is 
strongest  when  the  ball  is  first  thrown  ;  this  force,  as  it  pro- 
ceeds, being  weakened  by  the  continued  resistance  of  the  air, 
the  stone,  therefore,  begins  by  moving  in  a  horizontal  direc- 
tion ;  but  as  the  stronger  powers  prevail,  the  direction  of  the 
ball  will  gradually  change  from  a  horizontal,  to  a  perpendicu- 
lar line.  Projection  alone,  would  drive  the  ball  A,  to  B,. 
(fig.  3)  gravity  would  bring  it  to  C  ;  therefore,  when  acted 
on  in  different  directions,  by  these  two  forces,  it  moves  be- 
tween, gradually  inclining  more  and  more  to  the  force  of 
gravity,  in  proportion  as  this  accumulates  ;  instead  therefore 
of  reaching  the  ground  at  D,  as  you  supposed  it  would,  it  falls 
somewhere  about  E. 

Caroline.  It  is  precisely  so  ;  look,  Emily,  as  1  throw 
this  ball  directly  upwards,  how  the  resistance  of  the  air  and 
gravity  conquers  projection.  Now  I  will  throw  it  upwards 
obliquely  :  see  the  force  of  projection  enables  it,  for  an  instant^ 


ox  COMPOUND  MOTION,  ^l 

to  act  in  opposition  to  that  of  gravity  ;  but  it  is  soon  brought 
down  again. 

Mrs,  -S.  The  curve-line  which  the  ball  has  described,  is 
called  in  geometry  a  parabola  ;  but  when  the  ball  is  thrown 
perpendicularly  upwards,  it  will  descend  perpendicularly  ; 
because  the  force  of  projection,  and  that  of  gravity,  are  in  the 
same  line  of  direction. 

We  have  noticed  the  centres  of  magnitude,  and  of  motion  ; 
but  I  have  not  yet  explained  to  you,  what  is  meant  by  the 
centre  of  gravity  ;  it  is  that  point  in  a  body,  about  which  all 
the  parts  exactly  balance  each  other ;  if  therefore,  that  point 
is  supported,  the  body  will  not  fall.  Do  you  understand 
this  ? 

Emily.  I  think  so,  if  the  parts  round  about  this  point  have 
an  equal  tendency  to  fall,  they  will  be  in  equilibrium,  and  as 
long  as  this  point  is  supported,  the  body  cannot  fall. 

Mrs,  B.  Caroline,  what  would  be  the  effect,  were  any 
other  point  of  the  body  alone  supported  ? 

Caroline.  The  surrounding  parts  no  longer  balancing 
each  other,  the  body,  I  suppose,  would  fall  on  the  side  at 
which  the  parts  are  heaviest. 

Mrs,  B,  Infallibly  ;  whenever  the  centre  of  gravity  is 
unsupported,  the  body  must  fall.  This  sometimes  happens 
with  an  overloaded  waggon  winding  up  a  steep  hill,  one  side 
of  the  road  being  more  elevated  than  the  other  ;  let  us  suppose 
it  to  slope  as  is  described  in  this  figure,  (plate  III.  fig.  4,)  we 
will  say,  that  the  centre  of  gravity  of  this  loaded  waggon  is  at 
the  point  A.  Nowj^our  eye  will  tell  you,  that  a  waggon  thus 
situated,  will  overset  ;  and  the  reason  is,  that  the  centre  of 
gravity  A,  is  not  supported  ;  for  if  you  draw  a  perpendicular 
line  from  it  to  the  ground  at  C,  it  does  not  fall  under  the 
waggon  within  the  wheels,  and  is  therefore  not  supported  by 
them. 

Caroline,  I  understand  that  perfectly  ;  but  what  is  the 
meaning  of  the  other  point  B  ? 

Mrs,  B,  Let  us,  in  imagination,  take  off  the  upper  part 
cf  the  load  ;  the  centre  of  gravity  will  then  change  its  situa- 
tion, and  descend  to  B,  as  that  will  now  be  the  point  about 
which  the  parts  of  the  less  heavily  laden  waggon  will  balance 
each  other.     Will  the  waggon  now  be  upset  ? 

Caroline,  No,  because  a  perpendicular  line  from  that 
point  falls  within  the  wheels  at  D,  and  is  supported  by  them  ; 
and  when  the  centre  of  gravity  is  supported,  the  body  will  not 
fall 


■  'H  ON  COMPOUND  MOTION. 

Emily,  Yet  I  should  not  much  like  to  })ass  a  waggon,  lu 
that  siuiation  ;  for,  as  you  see,  the  point  D  is  but  just  within 
the  left  wheel  ;  if  the  right  wheel  was  merely  raised,  by  pass- 
ing over  a  stone,  the  point  D  would  be  thrown  on  the  outside 
of  the  left  wheel,  and  the  waggon  would  upset. 

Caroline.  A  waggon,  or  any  carriage  whatever,  will  then 
be  most  firmly  supported,  when  the  centre  of  gravity  falls 
exactly  between  the  wheels ;  and  that  is  the  case  in  a  level 
road. 

Pray,  v/hereabouts  is  the  centre  of  gra\ity  of  the  human 
body  ? 

Mrs,  B,  Between  the  hips  ;  and  as  long  as  we  stand 
upright,  this  point  is  supported  by  the  feet ;  if  you  lean  on 
one  side,  you  will  find  that  you  no  longer  stand  firm.  A 
rope-dancer  performs  all  his  feats  of  agility,  by  dexterously 
supporting  his  centre  of  gravity  ;  whenever  he  finds  that  he 
is  in  danger  of  losing  his  balance,  he  shifts  the  heavy  pole, 
which  he  holds  in  his  hands,  in  order  to  throw  the  weight 
towards  the  side  that  is  deficient ;  and  thus  by  changing  the 
situation  of  the  centre  of  gravity,  he  restores  his  equilibrium. 

Caroline.  When  a  stick  is  poised  on  the  tip  of  the  finger, 
is  it  not  by  supporting  its  centre  of  gravity  ? 

Mrs.  B.  Yes  ;  and  it  is  because  the  centre  of  gravity  is 
not  supported,  that  spherical  bodies  roll  down  a  slope.  A 
sphere  being  perfectly  round,  can  touch  the  slope  but  by  a 
single  point,  and  that  point  cannot  be  perpendicularly  under 
the  centre  of  gravity,  and  therefore  cannot  be  supported,  as 
you  will  perceive  by  examining  this  figure,     (fig.  5.  plate  III.) 

Emily.  So  it  appears  ;  yet  I  have  seen  a  cyhnder  of  wood 
roll  up  a  slope  ;  how  is  that  contrived  ? 

Mrs.  B.  It  is  done  by  plugging  one  side  of  the  cylinder 
with  lead,  as  at  B.  (fig.  5.  plate  III.)  the  body  being  no  long- 
er of  an  uniform  density,  the  centre  of  gravity  is  removed 
from  the  middle  of  the  body  to  some  point  in  the  lead,  as  that 
substance  is  much  heavier  than  wood  ;  now  you  may  observe 
that  in  order  that  the  cylinder  may  roll  down  the  plane,  as  it 
is  here  situated,  the  centre  of  gravity  must  rise,  which  is  impos- 
sible ;  the  centre  of  gravity  must  always  descend  in  moving, 
and  will  descend  by  the  nearest  and  readiest  means,  which 
will  be  by  forcing  the  cylinder  up  the  slope,  until  the  centre 
of  gravity  is  supported,  and  then  it  stops. 

Caroline.  The  centre  of  gravity,  therefore,  is  not  always 
in  the  middle  of  a  body. 

Mrs,  B.     No,  that  point  we  have  called  the  centre   ot 


ON  COMPOUND  MOTION.  59 

inagnitude  ;  when  the  body  is  of  an  uniform  density  the  centre 
of  gravity  is  in  the  same  point ;  but  when  one  part  of  the  body- 
is  composed  of  heavier  materials  than  another  part,  the  centre 
of  gravity  being  the  centre  of  the  weight  of  the  body  can  no 
longer  correspond  with  the  centre  of  magnitude.  Thus  you 
see  the  centre  of  gravity  of  this  cylinder  plugged  with  lead; 
cannot  be  in  the  same  spot  as  the  centre  of  magnitude. 

Emily,  Bodies,  therefore,  consisting  but  of  one  kind  of 
substance,  as  wood,  stone,  or  lead,  and  whose  densities  are 
consequently  uniform,  must  stand  more  firmly,  and  be  more 
difficult  to  overset,  than  bodies  composed  of  a  variety  of  sub- 
stances, of  different  densities,  which  ma^  throw  the  centre  of 
gravity  on  one  side. 

Mrs,  B,  Yes ;  but  there  is  another  circumstance  which 
more  materially  affects  the  firmness  of  their  position,  and  that 
is  their  form.  Bodies  that  have  a  narrow  base  are  easily- 
upset,  for  if  they  are  the  least  inclined,  their  centre  is  no 
longer  supported,  as  you  may  perceive  in  fig.  6. 

Caroline,  I  have  often  observed  with  what  difficulty  a 
person  carries  a  single  pail  of  water  ;  it  is  owing,  I  suppose, 
to  the  centre  of  gravity  being  thrown  on  one  side,  and  the 
opposite  arm  is  stretched  out  to  endeavor  to  bring  it  back,  to 
its  original  situation  ;  but  a  pail  hanging  on  each  arm  is  car- 
ried without  difficulty,  because  they  balance  each  other,  and 
:the  centre  of  gravity  remains  supported  by  the  feet. 

Mrs,  B,  Very  well ;  I  have  but  one  more  remark  to 
-make  on  the  centre  of  gravity,  v/hich  is,  that  when  two  bodies 
are  fastened  together,  by  a  line,  string,  chain,  or  any  power 
whatever,  they  are  to  be  considered  as  forming  but  one  body  ; 
if  the  two  bodies  be  of  equal  weight,  the  centre  of  gravity  will 
be  in  the  middle  of  the  line  which  unites  them,  (fig.  7)  but  if 
one  be  heavier  than  the  other,  the  centre  of  gravity  will  be 
proportionally  nearer  the  heavy  body  than  the  light  one. 
(fig.  8.)  If  you  were  to  carry  a  rod  or  pole  with  an  equal 
weight  fastened  at  each  end  of  it,  you  would  hold  it  in  the 
middle  of  the  rod,  in  order  that  the  weights  should  balance 
each  other  ;  whilst  if  it  had  unequal  weights  at  each  end  you 
would  hold  it  nearest  the  greater  weight,  to  make  them  balance 
each  other, 

Emily,  And  in  both  cases  we  should  sup])ort  the  centre 
of  gravity ;  and  if  one  weight  be  very  considerably  larger 
than  the  other,  the  centre  of  gravity  will  be  thrown  out  of  the 
rod  into  the  heaviest  weight,     (fig.  9.) 

Mrs.  B,     Undoubtedly. 


CONVERSATION  V. 


ON  THE  MECHANICAL  POWERS. 

Of  the  power  of  Machines  ;  Of  the  Lever  in  General ;  Of 
the  Lever  of  the  First  Kind,  having  the  Fulcrum  between 
the  Power  and  the  Weight ;  Of  the  Lever  of  the  Second 
Kind,  having  the  Weight  between  the  Po2ver  and  the 
Fidcrum  ;  Of  the  Lever  of  the  Third  Kind^  having  the 
Power  beticeen  the  Fulcrum  and  the  Weight, 


MRS.B. 

We  may  now  proceed  to  examine  the  mechanical  powers  ; 
they  are  six  in  number,  one  or  more  of  which  enters  into  the 
composition  of  every  machine.  The  lever,  the  pulley,  the 
wheel  and  axle,  the  inclined  plane,  the  wedge,  and  the  screw. 

In  order  to  understand  the  power  of  a  machine,  there  are 
four  things  to  be  considered.  1st.  The  power  that  acts  : 
this  consists  in  the  effort  of  men  or  horses,  of  weights,  springs, 
steam,  &;c. 

2dly.  The  resistance  which  is  to  be  overcome  by  the  pow- 
er ;  this  is  generally  a  weight  to  be  moved.  The  power  must 
always  be  superior  to  the  resistance,  otherwise  the  machine 
could  not  be  put  in  motion. 

Caroline,  If  for  instance  the  resistance  of  a  carriage  was 
greater  than  the  strength  of  the  horses  employed  to  draw  it, 
they  would  not  be  able  to  make  it  move. 

Mrs.  B.  3dly.  We  are  to  consider  the  centre  of  motion, 
or  as  it  is  termed  in  mechanics,  the  fulcrum  ;  this  you  may- 
recollect  is  the  point  about  which  all  the  parts  of  the  body- 
move  ;  and  lastly,  the  respective  velocities  of  the  power,  and 
of  the  resistance. 

Emily.     That  must  depend  upon  their  respective  distan- 


Tiq.    1. 


PL  ATI:  IV 


MT 


ON  THE  RIECHANICAL  POWERS,  6i 

t:es  from  the  axis  of  motion  ;  as  we  observed  in  the  motion  of 
the  vanes  of  the  windmill. 

MiS.  B,  We  shall  now  examine  the  power  of  the  lever. 
The  lever  is  an  inflexible  rod  or  beam  of  any  kind,  that  is  to 
say,  one  which  will  not  bend  in  any  direction.  For  instance, 
the  steel  rod  to  which  these  scales  are  suspended  is  a  lever, 
and  the  point  in  which  it  is  supported  the  fulcrum,  or  centre 
of  motion  ;  now,  can  you  tell  me  why  the  two  scales  are  in 
equilibrium  ? 

Caroline,  Being  both  empty,  and  of  the  same  weight, 
they  balance  each  other. 

Emily.  Or,  more  correctly  speaking,  because  the  centre 
of  gravity  common  to  both  is  supported. 

Mrs,  B.  Very  v/ell ;  and  which  is  the  centre  of  gravity  of 
this  pair  of  scales  ?  (fig.  1.  plate  IV.) 

Emily,  You  have  told  us  that  when  two  bodies  of  equal 
weight  were  fastened  together,  the  centre  of  gravity  was  in  the 
middle  of  the  line  that  connected  them  ;  the  centre  of  gravity 
of  the  scales  must  therefore  be  in  the  fulcrum  F  of  the  lever 
which  unites  the  two  scales  ;  and  corresponds  with  the  centre 
of  motion. 

Caroline.  But  if  the  scales  contained  different  weights, 
the  centre  of  gravity  would  no  longer  be  in  the  fulcrum  of  the 
lever,  but  removed  towards  that  scale  which  contained  the 
heaviest  weight ;  and  since  that  point  would  no  longer  be 
supported,  the  heav}^  scale  would  descend  and  out-weigh  the 
other. 

Mrs.  B.  True  ;  but  tell  me,  can  you  imagine  any  mode 
by  which  bodies  of  different  weights  can  be  made  to  balance 
each  other,  either  in  a  pair  of  scales,  or  simply  suspended  to 
the  extremities  of  the  lever  ?  for  the  scales  are  not  an  essen- 
tial part  of  the  machine,  they  have  no  mechanical  power, 
and  are  used  merely  for  the  convenience  of  containing  the 
substance  to  be  weighed. 

Caroline.  What  !  make  a  light  body  balance  a  heavy 
one  ?  I  cannot  conceive  that  possible. 

Mrs.  B.  The  fulcrum  of  this  pair  of  scales  (fig.  2.)  is 
moveable,  you  see  ;  I  can  take  it  off  the  prop,  and  fasten  it 
on  again  in  another  part  ;  this  part  is  now  become  the  ful- 
crum, but  it  is  no  longer  in  the  centre  of  the  lever. 

Caroline.     And  the  scales   are  no   longer  true  ;  for  that 
which  hangs  on  the  longest  side  of  the  lever  descends. 
6 


62  ON  THE  MECHANICAL  POWEHS. 

Mrs,  B,  The  two  parts  of  the  lever  divided  by  the  fulcrum 
are  called  its  arms,  you  should  therefore  say  the  longest  arm, 
not  the  longest  side  of  the  lever.  These  arms  are  likewise 
frequently  distinguished  by  the  appellations  of  the  acting  and 
the  resisting  part  of  the  lever. 

Your  observation  is  true  that  the  balance  is  now  destroy- 
ed ;  but  it  will  answer  the  purpose  of  enabling  you  to  com- 
prehend the  power  of  a  lever  when  the  fulcrum  is  not  in  the 
centre. 

Emily.  This  would  be  an  excellent  contrivance  for  those 
who  cheat  in  the  weight  of  their  goods  ;  by  making  the  ful- 
crum a  little  on  one  side,  and  placing  the  goods  in  the  scale 
which  is  suspended  to  the  longest  arm  of  the  lever,  they 
would  appear  to  weigh  more  than  they  do  in  reality. 

Mrs,  B,  You  do  not  consider  how  easily  the  fraud  would 
be  detected  ;  for  on  the  scales  being  emptied  they  would  not 
hang  in  equilibrium. 

Emily.  True  ;  I  did  not  think  of  that  circumstance. 
But  I  do  not  understand  why  the  longest  arm  of  the  lever 
should  not  be  in  equilibrium  with  the  other  ? 

Caroline.  It  is  because  it  is  heavier  than  the  shortest  arm  ; 
the  centre  of  gravity,  therefore,  is  no  longer  supported. 

Mrs.  B.  You  are  right  ;  the  fulcrum  is  no  longer  in  the 
centre  of  gravity  ;  but  if  we  can  contrive  to  make  the  fulcrum 
in  its  present  situation  become  the  centre  of  gravity,  the 
scales  will  again  balance  each  other  ;  for  you  recollect  that 
the  centre  of  gravity  is  that  point  about  which  every  part  of 
the  body  is  in  equilibrium. 

Emily.  It  has  just  occurred  to  me  how  this  may  be 
accomplished  ;  put  a  great  weight  into  the  scale  suspended 
to  the  shortest  arm  of  the  lever,  and  a  smaller  one  into  that 
suspended  to  the  longest  arm.  Yes,  I  have  discovered  it — 
look,  Mrs.  B.,  the  scale  on  the  shortest  arm  will  carry  Slbs., 
and  that  on  the  longest  arm  only  one,  to  restore  the  balance. 

Mrs.  B.  You  see,  therefore,  that  it  is  not  so  impracticable 
as  you  imagined  to  make  a  heavy  body  balance  a  light  one  ; 
and  this  is  in  fact  the  means  by  which  you  thought  an  impo- 
sition in  the  weight  of  goods  might  be  effected,  as  a  weight  of 
ten  or  twelve  ounces  might  thus  be  made  to  balance  a  pound 
of  goods.  Let  us  now  take  off  the  scales  that  we  may 
consider  the  lever  simply  ;  and  in  this  state  you  see  that  the 


ON  THE  i^rECHANICAL  POWERS.  6S 

(ulcriim  is  no  longer  the  centre  of  gravity  ;  but  it  is,  and 
must  ever  be^  the  centre  of  motion,  as  it  is  the  only  point 
which  remains  at  rest,  while  the  otlier  parts  move  about  it. 

Caroline.  It  now  resembles  the  two  opposite  vanes  of 
a  windmill,  and  the  fulcrum  the  point  round  which  they 
move. 

Mrs.  B.  In  describing  the  motion  of  those  vanes,  you 
may  recollect  our  observing  that  the  farther  a  body  is  from 
the  axis  of  motion,  the  greater  is  its  velocity. 

Caroline,     That  I  remember  and  understood  perfectly. 

Mrs.  B.  You  comprehend  then,  that  the  extremity  of 
the  longest  arm  of  a  lever  must  move  with  greater  velocity 
than  that  of  the  shortest  arm  ? 

Emili/,  No  doubt,  because  it  is  farthest  from  the  centre  of 
motion.  And  pray,  Mrs.  B.,  when  my  brothers  play  at 
see-saw,  is  not  the  plank  on  which  they  ride  a  kind  of  lever  ? 

Mrs.  B.  Certainly  ;  the  log  of  wood  v/hich  supports  it 
from  the  ground  is  the  fulcrum,  and  those  who  ride  represent 
the  power  and  the  resistance  at  each  end  of  the  lever.  And 
have  you  not  observed  that  when  those  who  ride  are  of  equal 
weight,  the  plank  m^ust  be  supported  in  the  middle  to  make 
the  two  arms  equal  ;  whilst  if  the  persons  differ  in  weight, 
the  plank  must  be  drawn  a  little  further  over  the  prop,  to 
make  the  arms  unequal,  and  the  lightest  person  who  repre- 
sents the  resistance,  must  be  placed  at  the  extremity  of  the 
longest  arm. 

Caroline.  That  is  always  the  case  when  I  ride  on  a  plank 
with  my  youngest  brother ;  I  have  observed  also  that  the 
lightest  person  has  the  best  ride,  as  he  moves  both  further  and 
quicker  ;  and  I  now  understand  that  it  is  because  he  is  more 
distant  from  the  centre  of  motion. 

Mrs.  B.  The  greater  velocity  with  which  your  little 
brother  moves,  renders  his  momentum  equal  to  yours. 

Caroline.  Yes  ;  I  have  i\ie  most  gravity,  he  the  greatest 
velocity  ;  so  that  upon  the  whole  our  momentums  are  equal. 
But  you  said,  Mrs.  B.,  that  the  power  should  be  greater  than 
the  resistance  to  put  the  machine  in  motion  ;  how  then  can 
the  plank  move  if  the  momentums  of  the  persons  who  ride 
are  equal. 

Mrs.  B.  Because  each  person  at  his  descent  touches  the 
ground  with  his  feet ;  the  reaction  of  which  gives  him  an 
impulse  which  increases  his  velocity  ;  this  spring  is  requisite 
to  destroy  the  equilibrium  of  the  power  and  the  resistance^ 


M  ON  TKE  MECHANICAL  POWERS. 

Otherwise^  the  plank  would  not  move.     Did  you  ever  observe 
that  a  lever  describes  the  arc  of  a  circle  in  its  motion  ? 

Emily.  No  ;  it  appears  to  me  to  rise  and  descend  per- 
pendicularly ;  at  least  I  always  thought  so. 

Mrs,  B.  I  believe  I  must  make  a  sketch  of  you  and  your 
brother  riding  on  a  plank;  in  order  to  convince  you  of  your 
error,  (fig.  4.  pi.  IV.)  You  may  now  observe  that  a  lever 
can  move  only  round  the  fulcrum,  since  that  is  the  centre  of 
motion  ;  it  would  be  impossible  for  you  to  rise  perpendicu- 
larly to  the  point  A,  or  for  your  brother  to  descend  in  a 
straight  line  to  the  point  B  ;  you  must  in  rising  and  he  in 
descending  describe  arcs  of  your  respective  circles.  This 
drawing  shows  you  also  how  much  superior  his  velocity 
must  be  to  yours  ;  for  if  you  could  swing  quite  round,  you 
would  each  complete  your  respective  circles  in  the  same  time. 
Caroline,  My  brother's  circle  being  much  the  largest  he 
must  undoubtedly  move  the  quickest. 

Mrs,  B,     Now  tell  me,  do   you  think  that  your  brother 
could  raise  you  as  easily  without  the  aid  of  a  lever  ? 
Caroline,     Oh  no,  he  could  not  lift  me  off  the  ground. 
Mrs,  B,     Then  I  think  you  require  no  further  proof  of  the 
power  of  a  lever,  since  you  see  what  it  enables  your  brother 
to  perform. 

Caroline.  I  now  understand  what  you  meant  by  saying, 
tliat  in  mechanics,  motion  was  opposed  to  matter,  for  it  is  my 
brother's  velocity  which  overcomes  my  weight. 

M)^s,  B,  You  may  easily  imagine,  what  enormous  weights 
may  be  raised  by  levers  of  this  description,  for  the  longer  the 
acting  part  of  the  lever  in  comparison  to  the  resisting  part, 
the  greater  is  the  effect  ptoduced  by  it ;  because  the  greater 
is  the  velocity  of  the  power  compared  to  that  of  the  weight. 

There  are  three  clifierent  kinds  of  levers ;  in  the  first  the 
fulcrum  is  between  the  power  and  the  weight. 

Caroline,  This  kind  then  com[>rehend3  the  several  levers 
you  have  described. 

Mrs,  B,  Yes,  when  in  levers  of  tlie  first  kind,  tlie  fulcrum 
^  is  equally  between  the  power  and  the  weight,  as  in  the  bal- 
ance, the  power  must  be  greater  than  the  weight,  in  order  to 
move  it ;  for  nothing  can  in  this  case  be  gained  by  velocity  ; 
the  two  arms  of  the  lever  being  equal,  the  velocity  of  their 
extremities  must  be  so  likewise.  The  balance  is  therefore  of 
no  assistance  as  a  mechanical  powder,  but  it  is  extremely  useful 
to  estimate  the  respective  v>^eights  of  bodies. 


ON  THE  MECHANICAL  POWERS.  65 

But  when  (fig.  5.)  the  fulcrum  F  of  a  lever  is  not  equally 
distant  from  the  power  and  the  weight,  and  that  the  power  P 
acts  at  the  extremity  of  the  longest  arm,  it  may  be  less  than 
the  weight  W,  its  deficiency  being  compensated  by  its  supe- 
rior velocity  ;  as  we  observed  in  the  see-smv, 

Emily.  Then  when  we  want  to  lift  a  great  weight,  wt 
must  fasten  it  to  the  shortest  arm  of  a  lever,  and  apply  our 
strength  to  the  longest  arm  ? 

Mrs,  jB.  If  the  case  will  admit  of  your  putting  the  end  of 
the  lever  under  the  weight,  no  fastening  will  be  required  ;  as 
you  will  perceive  by  stirring  the  fire. 

Emily,  Oh  yes  !  the  poker  is  a  lever  of  the  first  kind, 
the  point  where  it  rests  against  the  bars  of  the  grate  whilst  I 
am  stirring  the  fire,  is  the  fulcrum  ;  the  short  arm  or  resisting 
part  of  the  lever  is  employed  in  lifting  the  weight,  which  is 
the  coals,  and  my  hand  is  the  power  applied  to  the  longest 
arm,  or  acting  part  of  the  lever. 

Mrs,  B,  Let  me  hear,  Caroline,  whether  you  can  equally 
well  explain  this  instrument,  which  is  composed  of  two  levers, 
united  in  one  common  fulcrum. 

Caroline,     A  pair  of  scissors  ! 

Mrs,  B,  You  are  surprised,  but  if  you  examine  their 
construction,  you  will  discover  that  it  is  the  power  of  the  lever 
that  assists  us  in  cutting  with  scissors. 

Caroline,  Yes  ;  I  now  perceive  that  the  point  at  which 
the  two  levers  are  screwed  together,  is  the  fulcrum  ;  the 
handles,  to  which  the  power  of  the  fingers  is  applied,  are  the 
extremities  of  the  acting  part  of  the  levers,  and  the  cutting 
part  of  the  scissors,  are  the  resisting  parts  of  the  levers  : 
therefore,  the  longer  the  handles  and  the  shorter  the  points  of 
the  scissors,  the  more  easily  you  cut  with  them. 

Emily,  That  I  have  often  observed,  for  when  I  cut 
pasteboard  or  any  hard  substance,  I  always  make  use  of  that 
part  of  the  scissors  nearest  the  screw  or  rivet,  and  I  now 
understand  why  it  increases  the  power  of  cutting  ;  but  I  con- 
fess that  I  never  should  have  discovered  scissors  to  have  been 
double  levers  ;  and  pray  are  not  snufiers  levers  of  a  similar 
description  ? 

Mrs,  B,  Yes,  and  most  kinds  of  pincers  ;  the  great  power 
of  which  consists  in  the  resisting  part  of  the  lever  being  very 
short  in  comparison  of  the  acting  part. 

Caroline,.  And  of  what  nature  are  the  two  other  kinds  of 
levers  ? 

6* 


66  ON  THE  MECHANICAL  POWERS. 

Mrs.  B.     In  levers  of  the  second  kind,  the  weight,  instead 
of  being  at  one  end,  is   situated  between  the  power  and  the  . 
fulcrum,  (fig.  6.) 

Caroline,  The  weight  and  the  fulcrum  have  here  chang- 
ed places  ;  and  what  advantage  is  gained  by  this  kind  of 
lever  ? 

Mrs,  B.  In  moving  it,  the  velocity  of  the  power  must 
necessarily  be  greater  than  that  of  the  weight,  as  it  is  more 
distant  from  the  centre  of  the  motion.  Have  you  ever  seen 
your  brother  move  a  snow-ball  by  means  of  a  strong  stick, 
when  it  became  too  heavy  for  him  to  move  without  assistance  ? 
Caroline,  Oh  yes  ;  and  this  was  a  lever  of  the  second 
order  (fig.  7-)  ;  the  end  of  the  stick,  which  he  thrusts  under 
the  ball,  and  which  rests  on  the  ground,  becomes  the  fulcrum  ; 
the  ball  is  the  weight  to  be  moved,  and  the  power  his  hands 
applied  to  the  other  end  of  the  lever.  In  this  instance  there 
is  an  immense  difference  in  the  length  of  the  arms  of  the 
lever ;  for  the  weight  is  almost  close  to  the  fulcrum. 

Mrs,  B,  And  the  advantage  gained  is  proportional  to 
this  difference.  Fishermen's  boats  are  by  levers  of  this 
description  raised  from  the  ground  to  be  launched  into  the 
sea,  by  means  of  slippery  pieces  of  board  which  are  thrust 
under  the  keel.  The  most  common  example  that  we  have 
of  levers  of  the  second  kind  is  in  the  doors  of  our  apartments. 
EniiJij,  The  hinges  represent  the  fulcrum,  our  hands  the 
power  applied  to  the  other  end  of  the  lever  ;  but  where  is 
the  weight  to  be  moved  ? 

Mrs,  B,  The  door  is  the  weight,  and  it  consequently 
occupies  the  whole  of  the  space  between  the  power  and  the 
fulcrum.  Nutcrackers  are  double  levers  of  this  kind  ;  the 
hinge  is  the  fulcrum,  the  nut  the  resistance,  and  the  hands  the 
power. 

In  levers  of  the  third  kind  (fig.  8,),  the  fulcrum  is  again  at 
one  of  the  extremities,  the  weight  or  resistance  at  the  other, 
and  it  is  now  the  power  which  is  applied  between  the  fulcrum 
and  the  resistance. 

Emily,  The  fulcrum,  the  weight,  and  the  power,  then, 
each  in  their  turn,  occupy  some  part  of  the  middle  of  the  lever 
between  its  extremities.  But  in  this  third  kind  of  lever,  the 
weight  being  farther  from  the  centre  of  motion  than  the  pow- 
er, the  difficulty  of  raising  it  seems  increased  rather  than 
diminished. 

Mrs,  B.     That  is  very  true  ;  a  lever  of  this  kind  is  there- 


ox  THE  .MECHANICAL  POWERS.  6/ 

fore  never  used,  unless  absolutely  necessary,  as  is  the  case  in 
lifting  up  a  ladder  perpendicularly  in  order  to  place  it  against 
the  wall  ;  the  man  who  raises  it  cannot  place  his  hands  on 
the  upper  part  of  the  ladder,  the  power,  therefore,  is  necessa- 
rily placed  much  nearer  the  fulcrum  than  the  weight. 

Caroline.  Yes,  the  hands  are  the  power,  the  ground  the 
fulcrum,  and  the  upper  part  of  the  ladder  the  weight. 

Mrs.  B.  Nature  employs  this  kind  of  lever  in  the  struc- 
ture of  the  human  frame.  In  lifting  a  weight  with  the  hand, 
the  lower  part  of  the  arm  becomes  a  lever  of  the  third  kind  ; 
the  elbow^  is  the  fulcrum,  the  muscles  of  the  fleshy  part  of 
the  arm  the  power  ;  and  as  these  are  nearer  to  the  elbow  than 
the  hand,  it  is  necessary  that  their  power  should  exceed  the 
weight  to  be  raised. 

Emily.  Is  it  not  surprising  that  nature  should  have  fur- 
nished us  with  such  disadvantageous  levers  ? 

Mrs.  B.  The  disadvantage,  in  respect  to  power,  is  more 
than  counterbalanced  by  the  convenience  resulting  from  this 
structure  of  the  arm  ^  and  it  is  no  doubt  that  which  is  best 
adapted  to  enable  it  to  perform  its  various  functions. 

We  have  dwelt  so  long  on  the  lever,  that  we  must  reserve 
the  examination  of  the  other  mechanical  powers  to  our  next 
interview. 


CONVERSATION  V- 

CONTINUED. 


ON  THE  MECHANICAL  POWERS. 

Of  the  Fitlley  ;  Of  the   Wheel  and  Axle  ;   Of  the  Inclined^ 
Plane:  OftheJFedge;  Of  the  Screw, 


MRS.  B. 

The  pulley  is  the  second  mechanical  power  we  are  to 
examine.     You,  both,  I  suppose,  have  seen  a  pulley  ? 

Caroline.  Yes,  frequently  :  it  is  a  circular  and  flat  piece 
of  wood  or  metal,  with  a  string  which  runs  in  a  groove  round 
it ;  by  means  of  which,  a  weight  may  be  pulled  up  ;  thus 
pulleys  are  used  for  drawing  up  curtains. 

M7'S,  B.  Yes  ;  but  in  that  instance  the  pulleys  are  fixed, 
and  do  not  increase  the  power  to  raise  the  w^eights,  as  you 
will  perceive  by  this  figure,  (pi.  Y.  fig.  1.)  Observe  that  the 
fixed  pulley  is  on  the  same  principle  as  the  lever  of  a  pair  of 
scales,  in  which  the  fulcrum  F  being  in  the  centre  of  gravity, 
the  power  P  and  the  weight  W,  are  equally  distant  from  it, 
and  no  advantage  is  gained. 

Emily,  Certainly  ;  if  P  represents  the  power  employed 
to  raise  the  weight  W,  the  power  must  be  greater  than  the 
weight  in  order  to  move  it.  But  of  what  use  then  are  pulleys 
n  mechanics  ? 

Mrs,  B,  The  next  figure  represents  a  pulley  which  is 
not  fixed,  (fig.  2.)  and  thus  situated  you  will  perceive  that  it 
affords  us  mechanical  assistance.  In  order  to  raise  the  weight 
(W)  one  inch,  P,  the  power,  must  draw  the  strings  B  and  C 
one  inch  each  ;  the  whole  string  is  therefore  shortened  two 
inches,  while  the  weight  is  raised  only  one. 


4 


^^' 


PLATE    V. 


F 


■6  -6  m 


©N  THE  MECHANICAL  POWERS.  60 

Emily.  That  I  understand  :  if  P  drew  the  string  but  one 
inch,  the  weight  would  be  raised  only  half  an  inch,  because  it 
would  shorten  the  strings  B  and  C  half  an  inch  each,  and 
consequently  the  pulley  with  the  weight  attached  to  it,  can  be 
raised  only  half  an  inch. 

Caroline,  I  am  ashamed  of  my  stupidity  ;  but  I  confess 
that  I  do  not  understand  this  ;  it  appears  to  me  that  the 
weight  would  be  raised  as  much  as  the  string  is  shortened  by 
the  power. 

Mrs.  B.  I  will  endeavour  to  explain  it  more  clearly.  I 
fasten  this  string  to  a  chair  and  draw  it  towards  me  ;  I  have 
now  shortened  the  string,  by  the  act  of  drawing  it,  one  yard. 

Caroline.  And  the  chair,  as  I  supposed,  has  advanced 
one  yard. 

Mrs.  B.  This  exemplifies  the  nature  of  a  single  fixed 
pulley  only.  Now  unfasten  the  string,  and  replace  the  chair 
where  it  stood  before.  In  order  to  represent  the  moveable 
pulley,  we  must  draw  the  chair  forwards  by  putting  the  string 
round  it  ;  one  end  of  the  string  may  be  fastened  to  the  leg  of 
the  table,  and  I  shall  draw  the  chair  by  the  other  end  of  the 
string.  I  have  again  shortened  the  string  one  yard  ;  how 
much  has  the  chair  advanced  ? 

Caroline.  I  now  understand  it  ;  the  chair  represents  the 
weight  to  which  the  moveable  pulley  is  attached  ;  and  it  is 
very  clear  that  the  weight  can  be  drawn  only  half  the  length 
you  draw  the  string.  I  believe  the  circumstance  that  per- 
plexed me  was,  that  I  did  not  observe  the  difference  that 
resuhs  from  the  weight  being  attached  to  the  pulley,  instead 
of  being  fastened  to  the  string,  as  is  the  case  in  the  fixed  pulley. 

Emily.  But  I  do  not  3^et  understand  the  advantage  of 
pulleys  ;  they  seem  to  me  to  increase  rather  than  diminish 
the  difficulty  of  raising  weights,  since  you  must  draw  the 
string  double  the  length  that  you  raise  the  weight ;  whilst 
with  a  single  pulley,  or  without  any  pulley,  the  weight  is 
raised  as  much  as  the  string  is  shortened. 

Mrs.  B.  The  advantage  of  a  moveable  pidley  consists  in 
dividing  the  difficulty  ;  we  must  draw,  it  is  true,  twice  the 
length  of  the  string,  but  then  only  half  the  strength  is  required 
tliat  would  be  necessary  to  raise  the  weight  without  the 
assistance  of  a  moveable  pulley. 

Emily.  So  that  the  difficulty  is  overcome  in  the  same 
manner  as  it  would  be,  by  dividing  the  weight  into  two  equal 
parts,  and  raising  them  successively. 


70  ON  THE  MECHANICAL  POWERS. 

Mrs.  B,  Exactly.  You  must  observe,  that  with  a  move- 
able pulley  the  velocity  of  the  power  is  double  that  of  the 
weight,  since  the  power  P  (fig,  2.)  moves  two  inches,  whilst 
the  weight  W  moves  one  inch  ;  therefore  the  power  need  not 
be  more  than  half  the  weight  to  make  their  momentums  equal. 

Caroline,  Pulleys  act  then  on  the  same  principle  as  the 
lever,  the  deficiency  of  strength  of  the  power  being  compen- 
sated by  its  superior  velocity. 

Mr5.  B.  You  will  find,  that  all  mechanical  power  is 
founded  on  the  same  principle. 

Emily.  But  may  it  not  be  objected  to  pulleys,  that  a 
longer  time  is  required  to  raise  a  weight  by  their  aid  than 
without  it ;  for  what  you  gain  in  powder  you  lose  in  time  ? 

Mrs.  B.  That,  my  dear,  is  the  fundamental  law  in  me- 
chanics :  it  is  the  case  with  the  lever  as  well  as  the  pulley ; 
and  you  will  find  it  to  be  so  with  all  the  other  mechanical 
powers. 

Caroline.  I  do  not  see  any  advantage  in  the  mechanical 
powers  then,  if  what  we  gain  by  them  one  way  is  lost  another. 

Mrs.  B.  Since  we  are  not  able  to  increase  our  natural 
strength,  is  not  that  science  of  wonderful  utility,  by  means  of 
whiv'^h  we  may  reduce  the  resistance  or  weight  of  any  body  to 
the  level  of  our  strength  ?  This  the  mechanical  powers 
enable  us  to  accomplish,  by  dividing  the  resistance  of  a  body 
into  parts  which  we  can  successively  overcome.  It  is  true, 
as  you  observe,  that  it  requires  a  sacrifice  of  time  to  attain  this 
end,  but  you  mast  be  sensible  how  very  advantageously  it  is 
exchanged  for  power  :  the  utmost  exertion  we  can  make  adds 
but  little  to  our  natural  strength,  whilst  we  have  a  much  more 
unlimited  command  of  time.  You  can  now  understand,  that 
the  greater  the  number  of  pulleys  connected  by  a  string,  the 
more  easily  the  weight  is  raised,  as  the  difficulty  is  divided 
among  the  number  of  strings,  or  rather  of  parts  into  which  the 
string  is  divided  by  the  pulleys.  Several  pulleys  thus  connec- 
ted, form  what  is  called  a  system,  or  tackle  of  pulleys,  (fig. 
3.)  You  may  have  seen  them  suspended  from  cranes  to 
raise  goods  into  warehouses,  and  in  ships  to  draw  up  the 
sails. 

Emily.  But  since  a  fixed  pulley  affords  us  no  mechanical 
aid,  why  is  it  ever  used  ? 

Mrs.  B.  Though  it  does  not  increase  our  power,  it  is 
frequently  useful  for  altering  its  direction.  A  single  pulley 
enables  us  to  draw  up  a  curtain,  by  drawing  down  the  string 


ON  THE  MECHANICAL  POWERS.  ?! 

connected  with  it ;  and  we  should  be  much  at  a  loss  to  accom- 
plish this  simple  operation  without  its  assistance. 

Ca7'oline,  There  would  certainly  be  some  diificulty  in 
ascending  to  the  head  of  the  curtain,  in  order  to  draw  it  up. 
Indeed.  I  now  recollect  having  seen  workmen  raise  small 
w^eights  by  this  means,  whicli  seemed  to  answer  a  very  useful 
purpose. 

Mrs.  B.  In  shipping,  both  the  advantages  of  an  increase 
of  power  and  a  change  of  direction,  by  means  of  pulleys,  are 
united  :  for  the  sails  are  raised  up  the  masts  by  the  sailors  on 
deck,  from  the  change  of  direction  wdiich  the  pulley  effects, 
and  the  labour  is  facilitated  by  the  mechanical  power  of  a 
combination  of  pulleys. 

Emily,  But  the  pulleys  on  ship-board  do  not  appear  to 
me  to  be  united  in  the  manner  you  have  shown  us. 

Mrs.  B.  They  are,  I  believe,  generally  connected  as  des- 
cribed in  figure  4,  both  for  nautical,  and  a  variety  of  other 
purposes  ;  but  in  whatever  manner  pulleys  are  connected  by 
a  single  string,  the  mechanical  power  is  the  same. 

The  third  mechanical  powder  is  the  wheel  and  axle.  Let 
us  suppose  (plate  V.  fig.  5.)  the  weight  W  to  be  a  bucket 
of  water  in  a  w ell,  w^hich  we  raise  by  winding  the  rope,  to 
which  it  is  attached,  round  the  axle  ;  if  this  be  done  without 
a  wheel  to  turn  the  axle,  no  mechanical  assistance  is  received. 
The  axle  without  a  wheel  is  as  impotent  as  a  single  fixed 
pulley,  or  a  lever,  whose  fulcrum  is  in  the  centre  ;  but  add 
the  wheel  to  the  axle,  and  you  will  immediately  find  the 
bucket  is  raised  with  much  less  difficulty.  The  velocity  of  ^ 
the  circumference  of  the  wheel  is  as  much  greater  than  that  of 
the  axle,  as  it  is  further  from  the  centre  of  motion  :  for  the 
wheel  describes  a  great  circle  in  the  same  space  of  time  that 
the  axle  describes  a  small  one,  therefore  the  powder  is  increas- 
ed in  the  same  proportion  as  the  circumference  of  the  wheel 
is  greater  than  that  of  the  axle.  If  the  velocity  of  the  w  heel 
is  twelve  times  greater  than  that  of  the  axle,  a  pow  er  nearly 
twelve  times  less  than  the  weight  of  the  bucket  would  be  able 
to  raise  it. 

Emily.  The  axle  acts  the  part  of  the  shorter  arm  ol  the 
lever,  the  wheel  that  of  the  longer  arm. 

Caroline.  In  raising  water  there  is  commonly,  I  believe^ 
instead  of  a  wheel  attached  to  the  axle,  only  a  crooked  handle, 
which  answers  the  purpose  of  winding  the  rope  round  the 
axle,  and  thus  raising  the  bucket. 


72  eS  THE  MECHANICAL  POWERS. 

Mrs.  7>.  In  this  manner  (fig.  6.) ;  now  if  you  observe 
the  dotted  circle  which  the  handle  describes  in  winding  up 
the  rope,  you  will  perceive  that  the  branch  of  the  handle  A, 
which  is  united  to  the  axle,  represents  the  spoke  of  a  wheel, 
and  answers  the  purpose  of  an  entire  wheel ;  the  other 
branch  B  affords  no  mechanical  aid,  merely  serving  as  a  han- 
dle to  turn  the  wheel. 

Wheels  are  a  very  essential  part  to  most  machines  :  they 
are  employed  in  various  ways  ;  but,  when  fixed  to  the  axle, 
their  mechanical  power  is  always  the  same  ;  that  is,  as  the 
circumference  of  the  wheel  exceeds  that  of  the  axle,  so  much 
will  the  energy  of  its  power  be  increased. 

Caroline.  Then  the  larger  the  wh^el  the  greater  must  be 
its  effect. 

]\Irs.  B.  Certainly.  If  you  have  ever  seen  any  consid- 
erable mills  or  manufactures,  you  must  hive  admired  the 
immense  wheel,  the  revolution  of  which  puts  the  whole  of  the 
machinery  into  motion  ;  and  though  so  great  an  effect  is 
produced  by  it,  a  horse  or  two  has  sufficient  power  to  turn  it ; 
sometimes  a  stream  of  water  is  used  for  tliat  purpose,  but  of 
late  years,  a  steam-engine  has  been  found  both  the  most  pow- 
erful and  the  most  convenient  mode  of  turning  the  wheel. 

Caroline,  Do  not  the  vanes  of  a  windmill  represent  a 
wheel,  Mrs.  B.  ? 

Mrs.  B.  Yes ;  and  in  this  instance  we  have  the  advan- 
tage of  a  gratuitous  force,  the  wind,  to  turn  the  wheel.  One 
of  the  great  benefits  resulting  from  the  use  of  machinery  is, 
that  it  gives  us  a  sort  of  empire  over  the  powers  of  nature, 
and  enables  us  to  make  them  perform  the  labour  which  would 
otherwise  fall  to  the  lot  of  man.  When  a  current  of  wind,  a 
stream  of  water,  or  the  expansive  force  of  steam,  performs  our 
task,  we  have  only  to  superintend  and  regulate  their  operations. 

The  fourth  mechanical  power  is  the  inclined  plane  ;  this 
is  nothing  more  than  a  slope,  or  declivity,  frequently  used  to 
facilitate  the  drawing  up  of  weights.  It  is  not  difficult  to 
understand,  that  a  weight  may  much  more  easily  be  draw^n 
up  a  slope  than  it  can  be  raised  the  same  height  perpendicu- 
larly. But  in  this,  as  well  as  the  other  mechanical  powers, 
the  facility  is  purchased  by  a  loss  of  time  (fig.  7.) ;  for  the 
weight,  instead  of  moving  directly  from  A  to  C,  must  move 
from  B  to  C,  and  as  the  length  of  the  plane  is  to  its  height,  so 
much  is  the  resistance  of  the  weight  diminished. 


ON  THE  MECHANICAL  POWERS.  io 

Emily,  Yes  ;  for  the  resistance,  instead  of  being  confined 
to  the  short  line  A  C,  is  spread  over  the  long  line  B  C. 

Mrs.  B,  The  wedge,  which  is  the  next  mechanical  pow- 
er, is  composed  of  two  inclined  planes  (^fig.  8.) :  you  may 
have  seen  wood-cutters  use  it  to  cleave  wood.  The  resistance 
consists  in  the  cohesive  attraction  of  the  wood,  or  any  other 
body  which  the  wedge  is  employed  to  separate ;  and  the 
advantage  gained  by  this  power  is  in  the  proportion  of  half 
its  width  to  its  length  ;  for  whik  the  wedge  forces  asunder  the 
coherent  particles  of  the  wood  to  A  and  B,  it  penetrates 
downwards  as  far  as  C. 

Emily,  The  wedge,  then,  is  rather  a  compound  than  a 
distinct  mechanical  power,  since  it  is  composed  of  two 
inclined  planes. 

Mrs.  B.  It  is  so.  All  cutting  instruments  are  constructed 
upon  the  principle  of  the  inclined  plane,  or  the  wedge  :  those 
that  have  but  one  edge  sloped,  like  the  chisel,  may  be  referred 
to  the  inclined  plane  ;  whilst  the  axe,  the  hatchet,  and  the 
knife  (when  used  to  split  asunder)  are  used  as  wedges. 

Caroline.  But  a  knife  cuts  best  when  it  is  drawn  across 
the  substance  it  is  to  divide.  We  use  it  thus  in  cutting  meat, 
we  do  not  chop  it  to  pieces. 

Mrs.  B.  The  reason  of  this  is,  that  the  edge  of  a  knife  is 
really  a  very  fine  saw,  and  therefore  acts  best  when  used  like 
that  instrument. 

The  screw,  which  is  the  last  mechanical  power,  is  more 
complicated  than  the  others.  You  will  see  by  this  figure, 
(fig.  9.)  that  it  is  composed  of  two  parts,  the  screw  and  the 
nut.  The  screw  S  is  a  cylinder,  with  a  spiral  protuberance 
coiled  round  it,  called  the  thread  ;  the  nut  N  is  perforated  to 
contain  the  screw,  and  the  inside  of  the  nut  has  a  spiral 
groove  made  to  fit  the  spiral  thread  of  the  screw. 

Caroline.  It  is  just  like  this  little  box,  the  lid  of  which 
screws  on  the  box  as  you  have  described  :  but  what  is  this 
handle  which  projects  from  the  nut  ? 

Mrs,  B.  It  is  a  lever,  which  is  attached  to  the  nut,  without 
which  the  screw  is  never  used  as  a  mechanical  power  :  the 
nut  with  a  lever  L  attached  to  it,  is  commonly  called  a  winch. 
The  power  of  the  screw,  complicated  as  it  appears,  is  refera- 
ble to  one  of  the  most  simple  of  the  mechanical  powers  ; 
which  of  them  do  you  think  it  is  ? 

Caroline.  In  appearance,  it  most  resembles  the  wheel 
and  axle. 

7 


74  ON  THE  MECHANICAL  POWERS, 

Mrs.  B.  The  lever,  it  is  true,  has  the  effect  of  a  wheel,  as 
it  is  the  means  by  which  you  wind  the  nut  round  ;  but  the 
lever  is  not  considered  as  composing  a  part  of  the  screw, 
though  it  is  true,  that  it  is  necessarily  attached  to  it.  But 
observe,  that  the  lever,  considered  as  a  wheel,  is  not  fastened 
to  the  axle  or  screw,  but  moves  round  it,  and  in  so  doing,  the 
nut  either  rises  or  descends,  according  to  the  way  in  which 
you  turn  it. 

Emily,  The  spiral  thread  of  the  screw  resembles,  I  think, 
an  inclined  plane  :  it  is  a  sort  of  slope,  by  means  of  which 
the  nut  ascends  more  easily  than  it  would  do  if  raised  perpen- 
dicularly ;  and  it  serves  to  support  it  when  at  rest. 

Mrs,  B.  Very  well :  if  you  cut  a  slip  of  paper  in  the 
form  of  an  inclined  plane,  and  wind  it  round  your  pencil, 
which  will  represent  the  cylinder,  you  will  find  that  it  makes 
a  spiral  line,  corresponding  to  the  spiral  protuberance  of  the 
screw,  (fig.  10.) 

Emily,  Very  true;  the  nut  then  ascends  an  inclined 
plane,  but  ascends  it  in  a  spiral,  instead  of  a  straight  line  ;  the 
closer  the  thread  of  the  screw,  the  more  easy  the  ascent  ;  it 
is  like  having  shallow,  instead  of  steep  steps  to  ascend. 

Mrs.  B,  Yes,  excepting  that  the  nut  takes  no  steps,  it 
gradually  winds  up  or  down  ;  then  observe,  that  the  closer 
the  threads  of  the  screw,  the  greater  the  number  of  revolu- 
tions the  winch  must  make  ;  so  that  we  return  to  the  old 
principle, — what  is  saved  in  power  is  lost  in  time. 

Emily.  Cannot  the  power  of  the  screw  be  increased  also, 
by  lengthening  the  lever  attached  to  the  nut  ? 

Mrs.  B,  Certainly.  The  screw,  with  the  addition  of 
the  lever,  forms  a  very  powerful  machine,  employed  either 
for  compression  or  to  raise  heavy  weights.  It  is  used  by 
book-binders,  to  press  the  leaves  of  books  together ;  it  is  used 
also  in  cider  and  wine  presses,  in  coining,  and  for  a  variety  of 
other  purposes. 

All  machines  are  composed  of  one  or  more  of  these  six 
mechanical  powers  we  have  examined  :  I  have  but  one  more 
remark  to  make  to  you  relative  to  them,  which  is,  that  friction 
in  a  considerable  degree  diminishes  their  force,  allowance 
must  therefore  always  be  made  for  it  in  the  construction  of 
machinery. 

Caroline.  By  friction,  do  you  mean  one  part  of  the  ma- 
chine rubbing  against  another  part  contiguous  to  it. 

Mrs.  B.     Yes  ;  friction  is  the  resistance  which  bodies 


ON  THE  MECHANICAL  POWERS.  75 

meet  with  in  rubbing  against  each  other ;  there  is  no  such 
thing  as  perfect  smoothness  or  evenness  in  nature  :  poHshed 
metals,  though  they  wear  that  appearance,  more  than  any 
other  bodies,  are  far  from  really  possessing  it  ;  and  their 
inequalities  may  frequently  be  perceived  through  a  good 
magnifying  glass.  When,  therefore,  the  surfaces  of  the  two 
bodies  come  into  contact,  the  prominent  parts  of  the  one  will 
often  fall  into  the  hollow  parts  of  the  other,  and  occasion 
more  or  less  resistance  to  motion. 

Caroline.  But  if  a  machine  is  made  of  polished  metal,  as 
a  watch,  for  instance,  the  friction  must  be  very  trifling  ? 

Mrs.  B.  In  proportion  as  the  surfaces  of  bodies  are  well 
polished,  the  friction  is  doubtless  diminished  ;  but  it  is  always 
considerable,  and  it  is  usually  computed  to  destroy  one- 
third  of  the  power  of  a  machine.  Oil  or  grease  is  used  to 
lessen  friction  ;  it  acts  as  a  polish  by  filling  up  the  cavities  of 
the  rubbing  surfaces,  and  thus  making  them  slide  more  easily 
over  each  other. 

Caroline.  Is  it  for  this  reason  that  wheels  are  greased, 
and  the  locks  and  hinges  of  doors  oiled  ? 

Mrs.  B.  Yes  ;  in  these  instances  the  contact  of  the  rub- 
bing surfaces  is  so  close,  and  the  rubbing  so  continual,  that 
notwithstanding  their  being  polished  and  oiled,  a  considera- 
ble degree  of  friction  is  produced. 

There  are  two  kinds  of  friction  ;  the  one  occasioned  by 
the  sliding  of  the  flat  surface  of  a  body,  the  other  by  the 
rolling  of  a  circular  body  ;  the  friction  resulting  from  the 
first  is  much  the  most  considerable,  for  great  force  is  required 
to  enable  the  sliding  body  to  overcome  the  resistance  which 
the  asperities  of  the  surfaces  in  contact  oppose  to  its  motion, 
and  it  must  be  either  lifted  over,  or  break  through  them  ; 
whilst,  in  the  other  kind  of  friction,  the  rough  parts  roll  over 
each  other  with  comparative  facility  ;  hence  it  is,  that  wheels 
are  often  used  for  the  sole  purpose  of  diminishing  the  resist- 
ance of  friction. 

Emily.  This  is  one  of  the  advantages  of  carriage-wheels  ; 
is  it  not  ? 

Mrs.  B.  Yes  ;  and  the  larger  the  circumference  of  the 
wheel  the  more  readily  it  can  overcome  any  considerable 
obstacles,  such  as  stones,  or  inequalities  in  the  road.  When, 
in  descending  a  steep  hill,  we  fasten  one  of  the  wheels,  we 
decrease  the  velocity  of  the  carriage,  by  increasing  the 
friction. 


76  ON  THE  MECHANICAL  POWERS. 

Caroline,  That  is  to  say,  by  converting  the  rolling  fric- 
tion into  the  dragging  friction.  And  when  you  had  casters 
put  to  the  legs  of  the  table,  in  order  to  move  it  more  easily, 
you  changed  the  dragging  into  the  rolling  friction. 

Mrs,  B,  There  is  another  circumstance  which  we  have 
already  noticed,  as  diminishing  the  motion  of  bodies,  and 
which  greatly  affects  the  power  of  machines.  This  is  the 
resistance  of  the  medium  in  which  a  machine  is  worked.  All 
fluids,  whether  of  the  nature  of  air,  or  of  water,  are  called 
mediums  ;  and  their  resistance  is  proportioned  to  their  den- 
sity ;  for  the  more  matter  a  body  contains,  the  greater  the 
resistance  it  will  oppose  to  the  motion  of  another  body 
striking  against  it. 

Emily,  It  would  then  be  much  more  difficult  to  work  a 
machine  under  water  than  in  the  air  ? 

Mrs,  B,  Certainly,  if  a  machine  could  be  worked  in 
vacuOy  and  without  friction,  it  would  be  perfect ;  but  this  is 
unattainable  ;  a  considerable  reduction  of  power  must  there- 
fore be  allowed  for  the  resistance  of  the  air. 

We  shall  here  conclude  our  observations  on  the  mechan- 
ical powers.  At  our  next  meeting  I  shall  endeavour  to  give 
you  an  explanation  of  the  motion  of  the  heavenly  bodies. 


CONVERSATION  VI. 


CAUSES  OF  THE  EARTH'S  ANNUAL  MOTION. 

Of  the  Planets^  and  their  Motion  ;  Of  the  Diurnal  Motion 
of  the  Earth  and  Planets, 


CAROLINE. 

I  AM  come  to  you  to-day  quite  elated  with  the  spirit  of 
opposition,  Mrs.  B.  ;  for  I  have  discovered  such  a  powerful 
objection  to  your  theory  of  attraction,  that  I  doubt  whether 
even  your  conjuror  Newton,  with  his  magic  wand  of  attrac- 
tion, will  be  able  to  dispel  it. 

Mrs,  B.  Well,  my  dear,  pray  what  is  this  weighty  objec- 
tion ? 

Caroline.  You  say  that  bodies  attract  in  proportion  to 
the  quantity  of  matter  they  contain,  now  we  all  know  the  sun 
to  be  much  larger  than  the  earth  :  why,  therefore,  does  it  not 
attract  the  earth ;  you  will  not,  I  suppose,  pretend  to  say  that 
we  are  falling  towards  the  sun  ? 

Emily,  However  plausible  your  objection  appears,  Caro- 
line, I  think  you  place  too  much  reliance  upon  it  :  when  any 
one  has  given  such  convincing  proofs  of  sagacity  and  wisdom 
as  Sir  Isaac  Newton,  when  we  find  that  his  opinions  are  uni- 
rersally  received  and  adopted,  is  it  to  be  expected  that  any 
objection  we  can  advance  should  overturn  them  ? 

Caroline,  Yet  I  confess  that  I  am  not  inclined  to  yield 
implicit  faith  even  to  opinions  of  the  great  Newton :  for  what 
purpose  are  we  endowed  with  reason,  if  we  are  denied  the 
privilege  of  making  use  of  it,  by  judging  for  ourselves  ? 

MrSy  B,     It  is  reason  itself  which  teaches  us,  that  when 

we,  novices  in  science,  start  objections  to  theories  established 

by  men  of  acknowledged  wisdom,  we  should  be  diffident 

rather  of  our  own  than  of  their  opinion.     I  am  far  from  wish- 

7* 


78  CAUSES  OF  THK 

ing  to  lay  the  least  restraint  on  your  questions  ;  you  cannot 
be  belter  convinced  of  the  truth  of  a  system,  tlian  by  finding 
that  it  resists  all  your  attacks,  but  I  would  advise  you  not  to 
advance  your  objections  with  so  much  confidence,  in  order 
that  the  discovery  of  their  fallacy  may  be  attended  with  less 
mortification.  In  answer  to  that  you  have  just  proposed,  I 
can  only  say,  that  the  earth  really  is  attracted  by  the  sun. 

Caj'oline.  Take  care  at  least  that  we  are  not  consumed 
by  him,  Mrs.  B. 

Mrs.  B,  We  are  in  no  danger ;  but  our  magician  Newton, 
as  you  are  pleased  to  call  him,  cannot  extricate  himself  from 
this  difficulty  without  the  aid  of  some  cabalistical  figures, 
which  I  must  draw  for  him. 

Let  us  suppose  the  earth,  at  its  creation,  to  have  been 
projected  forwards  into  universal  space  :  we  know  that  if  no 
obstacle  impeded  its  course  it  would  proceed  in  the  same 
direction,  and  with  a  uniform  velocity  for  ever.  In  fig.  1 
plate  6.,  A  represents  the  earth,  and  S  the  sun.  We  shall 
suppose  the  earth  to  be  arrived  at  the  point  in  which  it  is 
represented  in  the  figure,  having  a  velocity  which  would  carry 
it  on  to  B  in  the  space  of  one  month  ;  whilst  the  sun's  attrac- 
tion would  bring  it  to  C  in  the  same  space  of  time.  Observe 
that  the  two  forces  of  projection  and  attraction  do  not  act  in 
opposition,  but  perpendicularly,  or  at  a  right  angle  to  each 
other.     Can  you  tell  me  now,  how  the  earth  will  move  ? 

Emily.  I  recollect  your  teaching  us  that  a  body  acted 
upon  by  two  forces  perpendicular  to  each  other  would  move 
in  the  diagonal  of  a  parallelogram  ;  if,  tlierefore,  I  complete 
the  parallelogram  by  drawing  the  lines  C  D,  B  D,  the  earth 
will  move  in  the  diagonal  A  D. 

Mrs.  B.  A  ball  struck  by  two  forces  acting  perpendicu- 
larly to  each  other,  it  is  true,  moves  in  the  diagonal  of  a 
parallelogram  ;  but  you  must  observe  that  the  force  of  attrac- 
tion is  continually  acting  upon  our  terrestrial  ball,  and  produ- 
cing an  incessant  deviation  from  its  course  in  a  right  line, 
which  converts  it  into  that  of  a  curve  line  ;  every  point  of 
w^hich  may  be  considered  as  constituting  the  diagonal  of  an 
infinitely  small  parallelogram. 

Let  us  detain  the  earth  a  moment  at  the  point  D,  and  con- 
sider how  it  will  be  aflected  by  the  combined  action  of  the  two 
forces  in  its  new  situation.  It  still  retains  its  tendency  to  fly 
off  in  a  straight  line ;  but  a  straight  line  would  now  carry  it 


PIATE    U 


^' 


^4^ 


0 


^' 


earth's  annual  motion.  79 

away  to  F,  whilst  the  sun  would  attract  it  in  the  direction 
D  S  ;  how  then  will  it  proceed  ? 

Emily.  It  will  go  on  in  a  curve  line,  in  a  direction  between 
that  of  the  two  forces. 

Mrs,  B.  In  order  to  know  exactly  what  course  the  earth 
will  follow,  draw  another  parallelogram  similar  to  the  first,  in 
which  the  line  D  F  describes  the  force  of  projection,  and  the 
line  D  S,  that  of  attraction  ;  and  you  will  find  that  the  earth 
will  proceed  in  the  curve  line  D  G. 

Caroline,  You  must  now  allow  me  to  draw  a  parallelo- 
gram, Mrs.  B.  Let  me  consider  in  w^hat  direction  will  the 
force  of  projection  now  impel  the  earth. 

Mrs,  B.  First  draw  a  line  from  the  earth  to  the  sun 
representing  the  force  of  attraction  ;  then  describe  the  force  of 
projection  at  a  right  angle  to  it. 

Caroline,  The  earth  will  then  move  in  the  curve  G  I,  of 
the  parallelogram  G  H  I  K. 

Mrs,  B,  You  recollect  that  a  body  acted  upon  by  two 
forces,  moves  through  a  diagonal  in  the  same  time  that  it 
would  have  moved  through  one  of  the  sides  of  the  parallelo- 
gram, were  it  acted  upon  by  one  force  only.  The  earth  has 
passed  through  the  diagonals  of  these  three  parallelograms  in 
the  space  of  three  months,  and  has  performed  one  quarter  of 
a  circle  ;  and  on  the  same  principle  it  will  go  on  till  it  has 
completed  the  whole  of  the  circle.  It  will  then  recommence 
a  course,  which  it  has  pursued  ever  since  it  first  issued  from 
the  hand  of  its  Creator,  and  which  there  is  every  reason  to 
suppose  it  will  continue  to  follow,  as  long  as  it  remains  in 
existence. 

Emily,  What  a  grand  and  beautiful  effect  resulting  from 
so  simple  a  cause  ! 

Caroline,  It  affords  an  example,  on  a  magnificent  scale, 
of  the  circular  motion  which  you  taught  us  in  mechanics. 
The  attraction  of  the  sun  is  the  centripetal  force,  which  con- 
fines the  earth  to  a  centre  ;  and  the  impulse  of  projection  the 
centrifugal  force,  which  impels  the  earth  to  quit  the  sun  and 
fly  off  in  a  tangent. 

Mrs,  B,  Exactly  so.  A  simple  mode  of  illustrating  the 
effect  of  these  combined  forces  on  the  earth,  is  to  cut  a  slip  of 
card  in  the  form  of  a  right  angle,  (fig.  2.  plate  YI.)  to  describe 
a  small  circle  at  the  angular  point  representing  the  earth,  and 
to  fasten  the  extremity  of  one  of  the  legs  of  the  angle  to  a 
fixed  point,  which  we  shall  consider  as  the  sun.     Thus  situa- 


30  CAUSES  OF  THE 

tedy  the  angle  will  represent  both  the  centrifugal  and  centri- 
petal forces  ;  and  if  you  draw  it  round  the  fixed  point,  you 
will  see  how  the  direction  of  the  centrifugal  force  varies, 
constantly  forming  a  tangent  to  the  circle  in  which  the  earth 
moves,  as  it  is  constantly  at  a  right  angle  with  the  centripetal 
force. 

Emihj,  The  earth,  then,  gravitates  towards  the  sun  with- 
out the  slightest  danger  either  of  approaching  nearer  or 
receding  further  from  it.  How  admirable  this  is  contrived  ! 
If  the  two  forces  which  produce  this  circular  motion  had  not 
been  so  accurately  adjusted,  one  would  ultimately  have  pre- 
vailed over  the  other,  and  we  should  either  have  approached 
so  near  the  sun  as  to  have  been  burnt,  or  have  receded  so  far 
from  it  as  to  have  been  frozen. 

Mrs.  B,  What  will  you  say,  my  dear,  when  I  tell  you 
that  these  two  forces  are  not,  in  fact,  so  proportioned  as  to 
produce  circular  motion  in  the  earth  ? 

Caroline.  You  must  explain  to  us,  at  least,  in  what  man- 
ner we  avoid  the  threatened  destruction. 

Mrs.  B.  Let  us  suppose  that  when  the  earth  is  at  A, 
(fig.  3.)  its  projectile  force  should  not  have  given  it  a  velocity 
sufficient  to  counterbalance  that  of  gravity,  so  as  to  enable 
these  powers  conjointly  to  carry  it  round  the  sun  in  a  circle  ; 
the  earth,  instead  of  describing  the  line  A  C,  as  in  the  former 
figure,  will  approach  nearer  the  sun  in  the  line  A  B. 

Caroline.  Under  these  circumstances,  I  see  not  what  is 
tc  prevent  our  approaching  nearer  and  nearer  the  sun  till  we 
fall  into  it :  for  its  attraction  increases  as  we  advance  towards 
it,  and  produces  an  accelerated  velocity  in  the  earth  which 
increases  the  danger. 

Mrs.  B.  And  there  is  yet  another  danger,  of  which  you 
are  not  aware.  Observe,  that  as  the  earth  approaches  the  sun, 
the  direction  of  its  projectile  force  is  no  longer  perpendicular 
to  that  of  attraction,  but  inclines  more  nearly  to  it.  When 
the  earth  reaches  that  part  of  its  orbit  at  B,  the  force  of  pro- 
jection would  carry  it  to  D,  which  brings  it  nearer  the  sun 
instead  of  bearing  it  away  from  it. 

Emily.  If,  then,  we  are  driven  by  one  power  and  drawn 
by  the  other  to  this  centre  of  destruction,  how  is  it  possible  for 
us  to  escape  ? 

Mrs.  B.  A  little  patience,  and  you  will  find  that  we  are 
not  without  resource.  The  earth  continues  approaching  tlie 
sun  with  a  uniformly  increasing  accelerated  motion,  till  it 


earth's  annual  3I0TI0N.  81 

ueaches  the  point  E  ;  in  what  direction  will  the  projectile 
ibrce  now  impel  it  ? 

Emily,  In  the  direction  E  F.  Here  then  the  two  forces 
act  perpendicularly  to  each  other,  and  the  earth  is  situated 
just  as  it  was  in  the  preceding  figure  ;  therefore,  from  this 
point,  it  should  revolve  round  the  sun  in  a  circle. 

Mrs,  B.  No,  all  the  circumstances  do  not  agree.  In  mo- 
tion round  a  centre,  3^ou  recollect  that  the  centrifugal  force 
increases  with  the  velocity  of  the  body,  or,  in  other  words, 
the  quicker  it  moves  the  stronger  is  its  tendency  to  fly  oil  in 
a  right  line.  When  the  earth,  therefore,  arrives  at  E,  its 
accelerated  motion  will  have  so  far  increased  its  velocity,  and 
consequently  its  centrifugal  force,  that  the  latter  will  prevail 
over  the  force  of  attraction,  and  drag  the  earth  away  from  the 
sun  till  it  reaches  G. 

Caroline,  It  is  thus,  then,  that  vre  escape  from  the  dan- 
gerous vicinity  of  the  sun  ;  and  in  proportion  as  we  recede 
from  it,  the  force  of  its  attraction,  and,  consequently,  the  velo- 
city of  the  earth's  motion  are  diminished. 

Mrs,  B,  Yes.  From  G  the  direction  of  projection  is 
towards  H,  that  of  attraction  towards  S,  and  the  earth  pro- 
ceeds between  them  with  a  uniformly  retarded  motion,  till  it 
has  completed  its  revolution.  Thus  you  see,  that  the  earth 
travels  round  the  sun,  not  in  a  circle,  but  an  ellipsis,  of  which 
the  sun  occupies  one  of  the  foci  ;  and  that  in  its  course  the 
earth  alternately  approaches,  and  recedes  from  it,  without  any 
danger  of  being  either  swallowed  up,  or  being  entirely  carried 
away  from  it. 

Caroline,  And  I  observe,  that  what  I  apprehended  to  be 
a  dangerous  irregularity,  is  the  means  by  which  the  most  per- 
fect order  and  harmony  are  produced  ! 

Emily,  The  earth  travels,  then,  at  a  very  unequal  rate, 
its  velocity  being  accelerated  as  it  approaches  the  sun,  and 
retarded  as  it  recedes  from  it. 

Mrs,  B,  It  is  mathematically  demonstrable,  that,  in  mov- 
ing round  a  point  towards  which  it  is  attracted,  a  body  passes 
over  equal  areas  in  equal  times.  The  whole  of  the  space 
contained  Vv'ithin  the  earth's  orbit,  is,  in  fig.  4.,  divided  into  a 
number  of  areas,  or  spaces,  1,  2,  3,  4,  &c.  all  of  which  are  of 
equal  dimensions,  though  of  very  different  forms ;  some  of 
them,  you  see,  are  long  and  narrow,  others  broad  and  short  : 
but  they  each  of  them  contain  an  equal  quantity  of  space. 
An  imaginary  line  drawn  from  the  centre  of  the  earth  to  that 


^2  CAUSES  OF  THE 

of  the  sun,  and  keeping  pace  with  the  earth  in  its  revolution, 
passes  over  equal  areas  in  equal  times  ;  that  is  to  say,  if  it  is 
a  month  going  from  A  to  B,  it  will  be  a  month  going  from  B 
to  C,  and  another  from  C  to  E,  and  so  on. 

Caroline.  What  long  journeys  the  earth  has  to  perform 
in  the  course  of  a  month,  in  one  part  of  her  orbit,  and  how 
short  they  are  in  the  other  part ! 

Mrs,  B.  The  inequality  is  not  so  considerable  as  appears 
in  this  figure ;  for  the  earth's  orbit  is  not  so  eccentric  as  it  is 
there  described  ;  and,  in  reality,  differs  but  litde  from  a  cir- 
cle ;  that  part  of  the  earth's  orbit  nearest  the  sun  is  called  its 
perihelion,  that  part  most  distant  from  the  sun  its  aphelion  ; 
and  the  earth  is  above  three  millions  of  miles  nearer  the  sun 
at  its  perihelion  than  at  its  aphelion. 

Emily.  I  think  I  can  trace  a  consequence  from  these  dif- 
ferent situations  of  the  earth  ;  is  it  not  the  cause  of  summer 
and  winter  ? 

Mrs.  B.  On  the  contrary  ;  during  the  height  of  summer, 
the  earth  is  in  that  part  of  its  orbit  which  is  most  distant  from 
the  sun,  and  it  is  during  the  severity  of  winter,  that  it  ap- 
proaches nearest  to  it. 

Emily.  That  is  very  extraordinary  ;  and  how  then  do 
you  account  for  the  heat  being  greatest,  when  we  are  most 
distant  from  the  sun  ? 

Mrs.  B.  The  difference  of  the  earth's  distance  from  the 
sun  in  summer  and  winter,  when  compared  with  its  total 
distance  from  the  sun,  is  but  ipconsiderable.  The  earth,  it  is 
true,  is  above  three  millions  of  miles  nearer  the  sun  in  winter 
than  in  summer  ;  but  that  distance,  however  great  it  at  first 
appears,  sinks  into  insignificance  in  comparison  of  95  mil- 
lions of  miles,  which  is  our  mean  distance  from  the  sun.  The 
change  of  temperature,  arising  from  this  difference,  would 
scarcely  be  sensible  ;  were  it  not  completely  overpowered  by 
other  causes  which  produce  the  variations  of  the  seasons  ;  but 
these  I  shall  defer  explaining,  till  we  have  made  some  further 
observations  on  the  heavenly  bodies. 

Caroline.  And  should  not  the  sun  appear  smaller  in  sum- 
mer, when  it  is  so  much  further  from  us  ? 

Mrs.  B.  It  actually  does  when  accurately  measured ;  but 
the  apparent  difference  in  size,  is,  I  believe,  not  perceptible  to 
the  naked  eye. 

Emily,  Then,  since  the  earth  moves  with  greatest  velo- 
city in  that  part  of  its  orbit  nearest  the  sun,  it  must  have 


EARTH'S  ANNUAL  MOTION.  83 

rdmpleted  its  journey  through  one  half  of  its  orbit  in  a  shortev 
time  than  the  other  half  ? 

Mrs.  B.  Yes,  it  is  about  seven  days  longer  performing 
the  summer-half  of  its  orbit,  than  the  winter-half. 

The  revolution  of  all  the  planets  round  the  sun  is  the  result 
of  the  same  causes,  and  is  performed  in  the  same  manner  as 
that  of  the  earth. 

Caroline.     Pray  what  are  the  planets  ? 

Mrs.  B,  They  are  those  celestial  bodies,  which  revolve 
iike  our  earth  about  the  sun  ;  they  are  supposed  to  resemble 
the  earth  also  in  many  other  respects  ;  and  we  are  led  by 
analogy  to  suppose  them  to  be  inhabited  worlds. 

Caroline.  I  have  heard  so  ;  but  do  you  not  think  such  an 
opinion  too  great  a  stretch  of  the  imagination  ? 

Mrs.  B.  Some  of  the  planets  are  proved  to  be  larger  than 
the  earth  ;  it  is  only  their  immense  distance  from  us,  which 
renders  their  apparent  dimensions  so  small.  Now,  if  we 
consider  them  as  enormous  globes,  instead  of  small  twinkhng 
spots,  we  shall  be  led  to  suppose,  that  the  Almighty  would 
not  have  created  them  merely  for  the  purpose  of  giving  us  a 
little  light  in  the  night,  as  it  was  formerly  imagined,  and  we 
should  find  it  more  consistent  with  our  ideas  of  the  Divine 
wisdom  and  beneficence  to  suppose  that  these  celestial  bodies, 
should  be  created  for  the  habitation  of  beings,  who  are,  like 
us,  blessed  by  his  providence.  Both  in  a  moral  as  well  as  a 
physical  point  of  view,  it  appears  to  me  more  rational  to  con- 
sider the  planets  as  worlds  revolving  round  the  sun  ;  and  the 
fixed  stars  as  other  suns,  each  of  them  attended  by  their 
respective  system  of  planets,  to  which  they  impart  their 
influence.  We  have  brought  our  telescopes  to  such  a  degree 
of  perfection,  that  from  the  appearances  which  the  moon 
exhibits  when  seen  through  them,  we  have  very  good  reason 
to  conclude,  that  it  is  a  habitable  globe,  for  though  it  is  true, 
that  we  cannot  discern  its  towns  and  people,  we  can  plainly 
perceive  its  mountains  and  valleys  ;  and  some  astronomers 
have  gone  so  far  as  to  imagine  they  discovered  volcanos. 

Emily.  If  the  fixed  stars  are  suns,  with  planets  revolving 
round  them,  why  should  we  not  see  those  planets  as  well  as 
their  suns  ? 

Mrs.  B.  In  the  first  place,  we  conclude  that  the  planets 
of  other  systems,  (like  those  of  our  own,)  are  much  smaller 
than  the  suns  which  give  them  light ;  therefore  at  so  great  a 
distance  as  to  make  the  suns  appear  like  fixed  stars,  the 


84  CAUSES  OF  TH£ 

planets  would  be  quite  invisible.  Secondiyj  the  light  of  the 
planets  being  only  reflected  light,  is  much  more  feeble  than 
that  of  the  fixed  stars.  There  is  exactly  the  same  difference 
as  between  the  light  of  the  sun  and  that  of  the  moon ;  the  first 
being  a  fixed  star,  the  second  a  planet. 

Emily,  But  if  the  planets  are  worlds  like  our  earth,  they 
are  dark  bodies  ;  and  instead  of  shining  by  night,  we  should 
see  them  only  by  day-light.  And  why  do  we  not  see  the 
fixed  stars  also  by  day-light  ? 

Mrs,  B,  Both  for  the  same  reason  ;  their  light  is  so  faint, 
compared  to  that  of  our  sun  reflected  by  the  atmosphere,  that 
it  is  entirely  efiaced  by  it :  the  light  emitted  by  the  fixed  stars 
may  probably  be  as  strong  as  that  of  our  sun,  at  an  equal 
distance  ;  but  being  so  much  more  remote,  it  is  diflfused  over 
a  greater  space,  and  is  consequently  proportionally  weakened. 

Caroline,  True  ;  I  can  see  much  better  by  the  light  of  a 
candle  that  is  near  me,  than  by  that  of  one  at  a  great  distance. 
But  I  do  not  understand  what  makes  the  planets  shine  ? 

Mrs,  B*  What  is  it  that  makes  the  steel  buttons  on  your 
brother's  coat  shine  ? 

Caroline,  The  sun.  But  if  it  was  the  sun  which  made 
the  planets  shine,  we  should  see  them  in  the  day-time  when 
the  sun  shone  upon  them  ;  or  if  the  faintness  of  their  light 
prevented  our  seeing  them  in  the  day,  we  should  not  see 
tliem  at  all,  for  the  sun  cannot  shine  upon  them  in  the  night. 

Mrs,  B,  There  you  are  in  error.  But  in  order  to  explain 
this  to  you,  I  must  first  make  you  acquainted  with  the  various 
motions  of  the  planets. 

You  know,  that  according  to  the  laws  of  attraction,  the 
planets  belonging  to  our  system  all  gravitate  towards  the  sun  ; 
and  that  this  force  combined  with  that  of  projection,  will 
occasion  their  revolution  round  the  sun,  in  orbits  more  or  less 
elliptical,  according  to  the  proportion  which  these  two  forces 
bear  to  each  other. 

But  the  planets  have  also  another  motion  ;  they  revolve 
upon  their  axes.  The  axis  of  a  planet  is  an  imaginary  line 
which  passes  through  its  centre,  and  on  which  it  turns  ;  and 
it  is  this  motion  which  produces  day  and  night.  With  that 
side  of  the  planet  facing  the  sun  it  is  day  ;  and  with  the 
opposite  side,  which  remains  in  darkness  it  is  night.  Our 
earth,  which  we  consider  as  a  planet,  is  24  hours  in  performing 
one  revolution  on  its  axis  ;  in  that  period  of  time,  therefore, 
we  have  a  day  and  a  night  ;  hence  this  revolution  is  called 


earth's  annual  motion.  85 

the  earth^s  diurnai  or  daily  motion  ;  and  it  is  this  revolution 
of  the  earth  from  west  to  east  which  produces  an  apparent 
motion  of  the  sun^  moon,  and  stars  in  a  contrary  direction. 

Let  us  now  suppose  ourselves  to  be  beings  independent  of 
any  planet,  travelling  in  the  skies,  and  looking  upon  the  earth 
in  the  same  point  of  view  as  upon  the  other  planets. 

Caroline.  It  is  not  flattering  to  us,  its  inhabitants,  to  see 
it  make  so  insignificant  an  appearance. 

Mrs.  B.  To  those  who  are  accustomed  to  contemplate 
it  in  this  light,  it  never  appears  more  glorious.  We  are 
taught  by  science  to  distrust  appearances  :  and  instead  of 
considering  the  planets  as  little  stars,  we  look  upon  them 
either  as  brilliant  suns  or  habitable  worlds,  and  we  consider 
the  whole  together  as  forming  one  vast  and  magnificent  sys- 
tem, worthy  of  the  Divine  hand  by  which  it  was  created. 

Emily.  I  can  scarcely  conceive  the  idea  of  this  immensity 
of  creation  ;  it  seems  too  sublime  for  our  imagination  : — and 
to  think  that  the  goodness  of  Providence  extends  over  mil- 
lions of  worlds  throughout  a  boundless  universe — Ah  !  Mrs. 
B.,  it  is  we  only  who  become  trifling  and  insignificant  beings 
in  so  magnificent  a  creation  ! 

Mrs.  B.  This  idea  should  teach  us  humility,  but  without 
producing  despondency.  The  same  Almighty  hand  which 
guides  these  countless  worlds  in  their  undeviating  course, 
conducts  with  equal  perfection  the  blood  as  it  circulates 
through  the  veins  of  a  fly,  and  opens  the  eye  of  the  insect  to 
behold  His  wonders.  Notwithstanding  this  immense  scale  of 
creation,  therefore,  w^e  need  not  fear  to  be  disregarded  or 
forgotten. 

But  to  return  to  our  station  in  the  skies.  We  were,  if  you 
recollect,  viewing  the  earth  at  a  great  distance,  in  appearance 
a  little  star,  one  side  illuminated  by  the  sun,  the  other  in 
obscurity.  But  would  you  believe  it,  Caroline,  many  of  the 
inhabitants  of  this  little  star  imagine  that  when  that  part 
which  they  inhabit  is  turned  from  the  sun,  darkness  prevails 
throughout  the  universe  merely  because  it  is  night  with  them  ; 
whilst,  in  reality,  the  sun  never  ceases  to  shine  upon  every 
planet.  When,  therefore,  these  little  ignorant  beings  look 
around  them  during  their  night,  and  behold  all  the  stars  shin- 
ing, they  cannot  imagine  why  the  planets,  which  are  dark 
bodies,  should  shine,  concluding,  that  since  the  sun  does  not 
illumine  themselves,  the  whole  universe  must  be  in  darkness. 
Caroline.     I  confess  that  I  was  one  of  these  ignorant  peo- 


86  CAUSES  OF  THE  EARTh's  ANNUAL  MOTION. 

pie ;  but  I  am  now  very  sensible  of  the  absurdity  of  such  an 
idea.  To  the  inhabitants  of  the  other  planets,  then,  we  must 
appear  as  a  little  star  ? 

Mrs.  B,  YeSj  to  those  which  revolve  round  our  sun  ; 
for  since  those  which  may  belong  to  other  systems  (and 
whose  existence  is  only  hypothetical,)  are  invisible  to  us,  it  is 
probable,  that  we  also  are  invisible  to  them. 

Emily,  But  they  may  see  our  sun  as  we  do  theirs,  in  ap- 
pearance a  fixed  star  ? 

Mrs,  B,  No  doubt,  if  the  beings  who  inhabit  those  planets 
are  endowed  with  senses  similar  to  ours.  By  the  same  rule, 
we  must  appear  as  a  moon,  to  the  inhabitants  of  our  moon  ; 
but  on  a  larger  scale,  as  the  surface  of  the  earth  is  about 
thirteen  times  as  large  as  that  of  the  moon. 

Emily,  The  moon,  Mrs.  B.,  appears  to  move  in  a  differ- 
ent direction,  and  in  a  different  manner  from  the  stars  ? 

Mrs,  B,  I  shall  defer  the  explanation  of  the  motion  of  the 
moon,  till  our  next  interview,  as  it  would  prolong  our  present 
lesson  too  much. 


CONVERSATION  VII 


ON  THE  PLANETS. 


fjf  the  Satellites  or  Moom  ;  Gravitij  diminishes  as  the 
Square  of  the  distance  ;  Of  the  Solar  System;  Of  Com- 
ets ;  Constellations.  Signs  of  the  Zodiac;  Of  Coperni- 
rus^  Newfon*  &'c. 


MRS.B. 

The  planets  are  distinguished  into  primary  and  secondary. 
Those  which  revolve  immediately  about  the  sun  are  called 
primary.  Many  of  these  are  attended  in  their  course  by 
smaller  planets,  which  revolve  round  them  :  these  are  called 
secondary  planets,  satellites,  or  moons.  Such  is  our  moon 
which  accompanies  the  ^arth,  and  is  carried  with  it  round  the 
sun. 

Emily,  How  then  can  you  reconcile  the  motion  of  the 
secondar}'  planets  to  the  laws  of  gravitation  ;  for  the  sun  is 
much  larger  than  any  of  the  primary  planets  ;  and  is  not  the 
power  of  gravity  proportional  to  the  quantity  of  matter  ? 

Caroline,  Perhaps  the  sun,  though  much  larger,  may  be 
less  dense  than  the  planets.  Fire  you  know  is  very  ligbt, 
and  it  may  contain  but  little  matter,  though  of  great  mag-.ii- 
tude. 

Mrs.  B.  We  do  not  know  of  what  kind  of  matter  the  sun 
is  made  ;  but  we  may  be  certain,  that  since  it  is  the  general 
centre  of  attraction  of  our  system  of  planets,  it  must  be  the 
body  which  contains  the  greatest  quantity  of  matter  in  tiat 
system. 

You  must  recollect,  that  the  force  of  attraction  is  not  only 
proportional  to  the  quantity  of  matter,  but  to  the  degree  of 


88  ON  THE  PLANETS. 

proximity  of  the  attractive  body  :  this  power  is  weakened,  by 
being  diffused,  and  diminishes  as  the  squares  of  the  distances 
increase.  The  square  is  the  product  of  a  number  muhiplied 
by  itseh^;  so  that  a  planet  situated  at  twice  the  distance  at 
which  we  are  from  the  sun  would  gravitate  four  times  less 
than  we  do  ;  for  the  product  of  two  multiplied  by  itself  is  four. 

Caroline,  Then  the  more  distant  planets  move  slower  in 
their  orbits;  for  their  projectile  force  must  be  proportioned 
to  that  of  attraction  ?  But  I  do  not  see  how  this  accounts  for 
the  motion  of  the  secondar}^  round  the  primary  planets j  in 
preference  to  the  sun  ? 

Emily.  Is  it  not  because  the  vicinity  of  the  primary 
planets  renders  their  attraction  stronger  than  that  of  the  sun  ? 

Mrs,  B,  Exactly  so.  But  since  the  attraction  between 
bodies  is  mutual,  the  primary  planets  are  also  attracted  by 
the  satellites,  which  revolve  round  them.  The  moon  attracts 
the  earth,  as  well  as  the  earth  the  moon  ;  but  as  the  latter  is 
the  smaller  body,  her  attraction  is  proportionally  less  ; 
therefore  neither  the  earth  revolves  round  the  moon,  nor  the 
moon  round  the  earth  ;  but  they  both  revolve  round  a  point, 
which  is  their  common  centre  of  gravity,  and  which  is  as 
much  nearer  the  earth  than  the  moon,  as  the  gravity  of  the 
former  exceeds  that  of  the  latter. 

Emily,  Yes,  I  recollect  your  saying,  that  if  two  bodies 
were  fastened  together  by  a  wire  or  bar,  their  common  centre 
of  gravity  would  be  in  the  middle  of  the  bar,  pro^'ided  the 
bodies  were  of  equal  weight  ;  and  if  they  differed  in  weight,  it 
would  be  nearer  the  larger  body.  If  then  the  earth  and  moon 
had  no  projectile  force  which  prevented  their  mutual  attrac- 
tion from  bringing  them  together,  they  would  meet  at  their 
common  centre  of  gravity. 

Caroline,  The  earth  then  has  a  great  variety  of  motions, 
it  revolves  round  the  sun,  upon  its  axis,  and  round  the  point 
towards  which  the  moon  attracts  it. 

Mrs.  B,  Just  so  ;  and  this  is  the  case  with  evory  planet 
which  is  attended  by  satellites.  The  complicated  effect  of 
this  variety  of  motions,  produces  certain  irregularities,  which, 
however,  it  is  not  necessary  to  notice  at  present. 

The  planets  act  on  the  sun  in  the  same  manner  as  they  are 
themselves  acted  on  by  their  satellites ;  for  attraction,  you 
must  remember,  is  always  mutual  ;  but  the  gravity  of  the 
planets  (even  when  taken  collectively)  is  so  trifling  compared 


PLA1\E    kh 


F.o.    L 


ILrschel 


ri^.  2. 


»j  Hfnv^  Ve7iits        Earth 

^"""&'    ^o      o      O 


IlerscJiel 


Ox\  THE  PLANETS,  '  89 

with  that  of  the  sun,  that  they  do  not  cause  the  latter  to  move 
so  much  as  one  half  of  his  diameter.  The  planets  do  not, 
therefore,  revolve  round  the  centre  of  the  sun,  but  round  a 
point  at  a  small  distance  from  its  centre,  about  which  the  sun 
also  revolves. 

Emily,     I  thought  the  sun  had  no  motion  ? 

Mrs.  B,  You  were  mistaken  ;  for  besides  that  which  I 
have  just  mentioned,  which  is  indeed  very  inconsiderable,  he 
revolves  on  his  axis  ;  this  motion  is  ascertained  by  observing 
certain  spots  w^hich  disappear,  and  re-appear  regularly  at 
stated  times. 

Caroline,  A  planet  has  frequently  been  pointed  out  to 
me  in  the  heavens  ;  but  I  could  not  perceive  that  its  motion 
differed  from  that  of  the  fixed  stars,  which  only  appear  to 
move. 

Mrs,  B,  The  great  distance  of  the  planets  renders  their 
motion  apparently  so  slow,  that  the  eye  is  not  sensible  of 
their  progress  in  their  orbit,  unless  we  watch  them  for  some 
considerable  length  of  time  :  in  different  seasons  they  appear 
in  different  parts  of  the  heavens.  The  most  accurate  idea  I 
can  give  you  of  the  situation  and  motion  of  the  planets,  will 
be  by  the  examination  of  this  diagram,  (plate  VII.  fig.  1.) 
representing  the  solar  system,  in  which  you  will  find  every 
planet  with  its  orbit  delineated. 

Emily,     But  the  orbits  here  are  all  circular,  and  you  said^- 
that  they  were  elliptical.     The  planets  appear  too,  to  be 
moving  round  the  centre  of  the  sun  ;  whilst  you  told  us  that 
they  moved  round  a  point  at  a  little  distance  from  thence. 

Mrs,  B,  The  orbits  of  the  planets  are  so  nearly  circular, 
and  the  common  centre  of  gravity  of  the  solar  system  so  near 
the  centre  of  the  sun,  that  these  deviations  are  scarcely  worth 
observing.  The  dimensions  of  the  planets,  in  their  true  pro- 
portions, you  will  find  delineated  in  fig.  2. 

Mercury  is  the  planet  nearest  the  sun  ;  his  orbit  is  conse- 
quently contained  within  ours  ;  but  his  vicinity  to  the  sun, 
occasions  his  being  nearly  lost  in  the  brilliancy  of  his  rays  ; 
and  when  we  see  the  sun,  he  is  so  dazzling,  that  very  accurate 
observations  cannot  be  made  upon  Mercury.  He  performs 
his  revolution  round  the  sun  in  about  87  days,  which  is  conse- 
quently the  length  of  his  year.  The  time  of  his  rotation  on 
his  axis  is  not  known  ;  his  distance  from  the  sun  is  computed 
to  be  37  millions  of  miles,  and  his  diameter  3180  miles, 
8* 


90  ON  THE  PLANETS. 

The  heat  of  this  planet  is  so  great,  that  water  cannot  exist 
tliere,  but  in  a  state  of  vapour,  and  metals  would  be  liquified.^ 

Caroline.     Oh,  what  a  dreadful  climate  ! 

Mrs,  B.  Though  we  could  not  live  there,  it  may  be  per- 
fectly adapted  to  other  beings  destined  to  inhabit  it. 

Venus,  the  next  in  the  order  of  planets,  is  68  millions  of 
miles  from  the  sun  ;  she  revolves  about  her  axis  in  23  hours 
and  21  minutes,  and  goes  round  the  sun  in  244  days  17  hours. 
The  orbit  of  Venus  is  also  within  ours  ;  during  one  half  of 
her  course  in  it,  we  see  her  before  sun-rise,  and  she  is  called 
the  morning  star  ;  in  the  other  part  of  her  orbit,  she  rises 
later  than  the  sun. 

Caroline.  In  that  case,  we  cannot  see  her,  for  she  must 
rise  in  the  day  time  ? 

Mrs.  B.  True  ;  but  when  she  rises  later  than  the  sun, 
sjie  also  sets  later  ;  so  that  we  perceive  her  approaching  the 
horizon  after  sun-set  :  she  is  then  called  Hesperus,  or  the 
evening  star.     Do  you  recollect  those  beautiful  lines  of  Milton : 

Now  came  still  evening  on,  and  twilight  gray 
Had  in  her  sober  livery  all  things  clad  ; 
Silence  accompanied ;  for  beast  and  bird, 
They  to  their  grassy  couch,  these  to  their  nests 
"Were  slunk,  all  but  the  wakeful  nightingale  ; 
She  all  night  long  her  amorous  descant  sung  ; 
Silence  waspleas'd  ;  now  glow'd  the  firmament 
With  living  sapphires.     Hesperus,  that  led 
The  starry  host,  rode  brightest,  till  the  moon 
Rising  in  clouded  majesty,  at  length 
Apparent  queen  unveil'd  her  peerless  light, 
And  o'er  the  dark  her  silver  mantle  threw. 

The  planet  next  to  Venus  is  the  Earth,  of  which  we  shall 
soon  speak  at  full  length.  At  present  I  shall  only  observe, 
that  we  are  95  millions  of  miles  distant  from  the  sun,  that  we 
perform  our  annual  revolution  in  365  days,  5  hours,  and  49 
minutes  ;  and  are  attended  in  our  course  by  a  single  moon. 

Next  follows  Mars.  He  can  never  come  between  us  and 
the  sun,  like  Mercury  and  Venus  ;  his  motion  is,  however, 
very  perceptible,  as  he  may  be  traced  to  different  situations  in 

*  The  intenseness  of  the  sun's  heat,  which  is  in  the  same  proportion  as 
his  light,  is  seven  times  as  great  in  Mercur}  as  with  us  ;  so  that  water 
there  would  be  carried  off  in  the  shape  of  steam,  for  by  experiments  with 
the  thermometer,  it  appears  that  a  heat  seven  times  greater  than  that  d"  the 
sun's  beams  in  summer  will  serve  to  make  water  boil. 


ON  THE  PLANETS.  91 

the  heavens;  his  distance  from  the  gun  is  144  miUions  of 
miles  ;  he  turns  round  his  axis  in  24  hours  and  39  minutes  ; 
and  he  performs  his  annual  revolution,  in  about  687  of  our 
days:  his  diameter  is  4120  miles.  Then  follow  four  very 
small  planets,  Juno,  Ceres,  Pallas,  and  Vesta,  which  have 
been  recently  discovered,  but  whose  dimensions  and  distances 
from  the  sun  have  not  been  very  accurately  ascertained.* 

Jupiter  is  next  in  order  :  this  is  the  largest  of  all  the  plan- 
ets. He  is  about  490  millions  of  miles  from  the  sun,  and 
completes  his  annual  period  in  nearly  12  of  our  years.  He 
turns  round  his  axis  in  about  ten  hours.  He  is  above  1200 
times  as  big  as  our  earth  ;  his  diameter  being  86^000  miles. 
The  respective  proportions  of  the  planets  cannot,  therefore, 
you  see,  be  conveniently  delineated  in  a  diagram.  He  is 
attended  by  four  moons. t 

The  next  planet  is  Saturn,  whose  distance  from  the  sun  is 
about  900  millions  of  miles  ;  his  diurnal  rotation  is  performed 
in  10  hours  and  a  quarter: — his  annual  revolution  in  nearly 
30  of  our  years.  His  diameter  is  79,000  miles.  This  planet 
is  surrounded  by  a  luminous  ring,  the  nature  of  which,  as- 
tronomers are  much  at  a  loss  to  conjecture  ;  he  has  seven 
moons.  Lastly,  we  observe  the  Georgium  Sidus,  discovered 
by  Dr.  Herschel,  and  which  is  attended  by  six  moons.  J 

Caroline.  How  charming  it  must  be  in  the  distant  planets, 
to  see  several  moons  shining  at  the  same  time  ;  I  think  I 
should  like  to  be  an  inhabitant  of  Jupiter  or  Saturn. 

Mrs.  B.  Not  long,  I  believe.  Consider  what  extreme 
cold  must  prevail  in  a  planet,  situated  as  Satarn  is,  at  nearly 
ten  times  the  distance  at  which  we  are  from  the  sun.  Then 
his  numerous  moons  are  far  from  making  so  splendid  an 

*These  anomalous  bodies,  so  unlike  the  other  primary  planets,  Dr. 
Herschel  has  denominated  Asteroids.  Probably  they  are  the  fragments 
of  some  planet  ;  or  perhaps  other  similar  bodies  abound  in  the  solar 
system,  though  they  have  hitherto,  from  their  sraallness  or  darkness, 
escaped  observation. 

-f  Jupiter  is  surrounded  by  cloudy  substances,  subject  to  frequent 
changes  in  their  situation  and  appearance,  called  Belts.  These  Belts  are 
sometimes  of  a  regular  form  ;  sometimes  interrupted  and  broken  ;  and 
sometimes  not  at  all  to  be  seen. 

t  This  ring  is  set  edgewise  round  it,  and  the  distance  of  the  rmg  from 
the  planet  is  equal  to  the  breadth  of  the  ring.  The  sun  shines  for  almost 
fifteen  of  our  years  together  on  the  northern  side  of  the  ring  ;  then  goes 
off,  and  shines  as  long  on  the  southern  side  of  it,  so  there  is  but  one  day 
and  one  night  on  each  side  of  the  ring,  in  the  time  of  Saturn's  whole 
revolution  about  the  sun,  whi«h  takes  up  almost  thirty  of  our  years. 


92  ON  THE  PLANETS. 

appearance  as  ours  ;  for  th^y  can  reflect  only  the  light  which 
they  receive  from  the  sun  ;  and  both  light  and  heat  decrease 
in  the  same  ratio  or  proportion  to  the  distances  as  gravity. 
Can  you  tell  me  now  how  much  more  light  we  enjoy  than 
Saturn. 

Caroline,  The  square  of  ten,  is  a  hundred  ;  therefore, 
Saturn  has  a  hundred  times  less — or  to  answer  your  question 
exactly,  we  have  a  hundred  times  more  light  and  heat  than 
Saturn — this  certainly  does  not  increase  my  wish  to  become 
one  of  the  poor  wretches  who  inhabit  that  planet.* 

Mrs,  B.  May  not  the  inhabitants  of  Mercury,  with  equal 
plausibility,  pity  us,  for  the  insupportable  coldness  of  our 
situation  ;  and  those  of  Jupiter  and  Saturn  for  our  intolerable 
heat  ?  The  Almighty  Power  w^hich  created  these  planets, 
and  placed  them  in  their  several  orbits,  has  no  doubt  peopled 
them  w^ith  beings  whose  bodies  are  adapted  to  the  various 
temperatures  and  elements  in  which  they  are  situated.  If  we 
judge  from  the  analogy  of  our  own  earth,  or  from  that  of  the 
great  and  universal  beneficence  of  Providence,  we  must  con- 
clude this  to  be  the  case. 

Caroline.     Are  not  comets  also  supposed  to  be  planets  ? 

Mrs.  B.  Yes,  they  are  ;  for  by  the  re-appearance  of  some 
of  them,  at  stated  times,  they  are  know^n  to  revolve  round  the 
sun,  but  in  orbits  so  extremely  eccentric,  that  they  disappear 
for  a  great  number  of  years.  If  they  are  inhabited,  it  must 
be  by  a  species  of  beings  very  different,  not  only  from  the 
inhabitants  of  this,  but  from  those  of  any  of  the  other  planets, 
as  they  must  experience  the  greatest  vicissitudes  of  heat  and 
cold  ;  one  part  of  their  orbit. being  so  near  the  sun,  that  their 
heat,  when  there,  is  computed  to  be  greater  than  that  of  red- 
hot  iron  ;  in  this  part  of  its  orbit,  the  comet  emits  a  luminous 
vapour,  called  the  tail,  which  it  gradually  loses  as  it  recedes 
from  the  sun  ;  and  the  comet  itself  totally  disappears  from 
our  sight,  in  the  more  distant  parts  of  its  orbit,  which  extends 
considerably  beyond  that  of  the  furthest  planet. 

The  number  of  comets  belonging  to  our  system,  cannot  be 
ascertained,  as  some  of  them  are  whole  centuries  before  they 
make  their  re-appearance.     The  numbers  that  are  known  by  . 
their  regular  re-appearance  is  only  three. 

Emily.     Pray,  Mrs.  B.  what  are  the  constellations  ? 

•  The  sun's  light  at  Saturn  is  1 0^0  times   as  great  as  the  light  of  the 
full  moon  is  to  us. 


PhAT^  1  I 


ON  THE  PLANETS.  93 

Mrs,  B,  They  are  the  fixed  stars,  which  the  ancients,  in 
order  to  recognize  them,  formed  into  groupes,  and  gave  the 
names  of  the  figures,  which  you  find  delineated  on  the  celestial 
globe.  In  order  to  show  their  proper  situations  in  the  heav- 
ens, they  should  be  painted  on  the  internal  surface  of  a  hollow 
sphere,  from  the  centre  of  which  you  should  view  them ;  you 
would  then  behold  them,  as  they  appear  to  be  situated  in  the 
heavens.  The  twelve  constellations,  called  the  signs  of  the 
zodiac,  are  those  w^hich  are  so  situated,  that  the  earth  in  its 
annual  revolution  passes  directly  between  them  and  the  sun. 
Their  names  are  Aries,  Taurus,  Gemini,  Cancer,  Leo,  Virgo, 
Libra,  Scorpio,  Sagittarius,  Capricornus,  Aquarius,  Pisces  ; 
the  whole  occupying  a  complete  circle,  or  broad  belt,  in  the 
heavens,  called  the  zodiac,  (plate  VIII.  fig,  1.)  Hence,  a 
right  line  drawn  from  the  earth,  and  passing  through  the  sun, 
v>^ould  reach  one  of  these  constellations,  and  the  sun  is  said  to 
be  in  that  constellation  at  which  the  line  terminates  :  thus, 
when  the  earth  is  at  A,  the  sun  would  appear  to  be  in  the 
constellation  or  sign  Aries  ;  when  the  earth  is  at  B,  the  sun 
would  appear  in  Cancer  ;  when  the  earth  was  at  C,  the  sun 
would  be  in  Libra  ;  and  when  the  earth  was  at  D,  the  sun 
would  be  in  Capricorn.  This  circle,  in  which  the  sun  thus 
appears  to  move,  and  which  passes  through  the  middle  of  the 
zodiac,  is  called  the  ecliptic. 

Caroline,  But  many  of  the  stars  in  these  constellations 
appear  beyond  the  zodiac. 

Mrs,  B,  We  have  no  means  of  ascertaining  the  distance 
of  the  fixed  stars.  When,  therefore,  they  are  said  to  be  in  the 
zodiac,  it  is  merely  implied,  that  they  are  situated  in  that 
direction,  and  that  they  shine  upon  us  through  that  portion  of 
the  heavens,  which  we  call  the  zodiac* 

Emihj,  But  are  not  those  large  bright  stars,  which  are 
called  stars  of  the  first  magnitude,  nearer  to  us,  than  those 
small  ones  which  vv^e  can  scarcely  discern  ? 

Mrs,  B.  It  may  be  so  ;  or  the  difference  of  size  and 
brilliancy  of  the  stars  may  proceed  from  their  difference  of 
dimensions  ;  this  is  a  point  which  astronomers  are  not  enabled 
to  determine.     Considering  them  as  suns,  I  see  no  reason 

*  An  easy  distinction  between  a  planet  and  a  f.xed  star  is  this — the 
torn-iCr  shines  with  a  steady  liglit,  but  the  latter  is  constantly  twinkling. 
\V hat  it  is  which  occasions  tliis  twinkling*  oi*  scintillation  of  a  star,  y^t 
remains  undecided. 


94  ON  THE  PLANETS. 

why  different  sunS  should  not  vary  in  dimensions  as  well  as 
the  planets  belonging  to  them. 

Emily,  Whai  a  wonderful  and  beautiful  system  this  is,  and 
how  astonishing  to  think  that  every  fixed  star  may  probably 
be  attended  by  a  similar  train  of  planets  ! 

Caroline,  You  will  accuse  me  of  being  very  incredulous, 
but  I  cannot  help  still  entertaining  some  doubts,  and  fearing 
that  there  is  more  beauty  than  truth  in  this  system.  It  cer- 
tainly may  be  so  ;  but  there  does  not  appear  to  me  to  be 
sufficient  evidence  to  prove  it.  It  seems  so  plain  and  obvious 
that  the  earth  is  motionless,  and  that  the  sun  and  stars  revolve 
round  it  ;  your  solar  system,  you  must  allow,  is  directly  in 
opposition  to  the  evidence  of  our  senses. 

Mrs,  B.  Our  senses  so  often  mislead  us,  that  we  should 
not  place  implicit  reliance  upon  them. 

Caroline,  On  what  then  can  we  rely,  for  do  we  not  receive 
all  our  ideas  through  the  medium  of  our  senses  ? 

Mrs,  B,  It  is  true  that  they  are  our  primary  source  of 
knowledge  ;  but  the  mind  has  the  power  of  reflecting,  judging, 
and  deciding  upon  the  ideas  received  by  the  organs  of  sense. 
This  faculty,  which  we  call  reason,  has  frequently  proved  to 
us,  that  our  senses  are  liable  to  err.  If  you  have  ever  sailed 
on  the  water,  with  a  very  steady  breeze,  you  must  have  seen 
the  houses,  trees,  and  every  object  move,  while  you  were 
sailing. 

Caroline,  I  remember  thinking  so,  when  I  was  very 
young  ;  but  I  now  know  that  their  motion  is  only  apparent. 
It  is  true  that  my  reason,  in  this  case,  corrects  the  error  of  my 
sight. 

Mrs,  B,  It  teaches  you  that  the  apparent  motion  of  the 
objects  on  shore,  proceeds  from  your  being  yourself  moving, 
and  that  you  are  not  sensible  of  your  own  motion,  because 
you  meet  with  no  resistance.  It  is  only  when  some  obstacle 
impedes  our  motion,  that  we  are  conscious  of  moving  ;  and  if 
you  were  to  close  your  eyes  when  you  were  sailing  on  calm 
water,  with  a  steady  wind,  you  would  not  perceive  that  you 
moved,  for  3^ou  could  not  feel  it,  and  you  could  see  it  only  by 
observing  the  change  of  place  of  the  objects  on  shore.  So  it 
is  with  the  motion  of  the  earth  ;  every  thing  on  its  surface, 
and  the  air  that  surrounds  it,  accompanies  it  in  its  revolution  ; 
it  meets  w^ith  no  resistance :  therefore,  hke  the  crew  of  a 
vessel  sailing  w^ith  a  fair  wind,  in  a  calm  sea,  we  are  insensible 
of  our  motion. 


ON  THE  PLANETS.  95 

Caroline,  But  the  principal  reason  why  the  crew  of  a 
vessel  in  a  calm  sea  do  not  perceive  their  motion,  is,  because 
they  move  exceedingly  slowly  ;  while  the  earth,  you  say, 
revolves  with  great  velocity. 

Mrs*  B.  It  is  not  because  they  move  slowly,  but  because 
they  move  steadily,  and  meet  with  no  irregular  resistances, 
that  the  crew  of  a  vessel  do  not  perceive  their  motion  ;  for 
they  would  be  equally  insensible  to  it,  with  the  strongest 
wind,  provided  it  were  steady,  that  they  sailed  with  it,  and 
that  it  did  not  agitate  the  water ;  but  this  last  condition,  you 
know,  is  not  possible,  for  the  wind  will  always  produce  waves 
whkh  offer  more  or  less  resistance  to  the  vessel,  and  then  the 
motion  becomes  sensible,  because  it  is  unequal. 

Caroline.  But,  granting  this,  the  crew  of  a  vessel  have  a 
proof  of  their  motion,  though  insensible,  which  the  inhabitants 
of  the  earth  cannot  have, — the  apparent  motion  of  the  objects 
on  shore. 

Mrs.  B.  Have  we  not  a  similar  proof  of  the  earth's  motion, 
in  the  apparent  motion  of  the  sun  and  stars.  Imagine  the 
earth  to  be  sailing  round  its  axis,  and  successively  passing  by 
every  star,  which,  like  the  objects  on  land,  we  suppose  to  be 
moving  instead  of  ourselves.  I  have  heard  it  observed  by  an 
aerial  traveller  in  a  balloon,  that  the  earth  appears  to  sink 
beneath  the  balloon,  instead  of  the  balloon  rising  above  the 
earth. 

It  is  a  law  which  we  discover  throughout  nature  and  worthy 
of  its  great  Author,  that  all  its  purposes  are  accomplished  by 
the  most  simple  means ;  and  what  reason  have  we  to  suppose 
this  law  infringed,  in  order  that  we  may  remain  at  rest,  while 
the  sun  and  stars  move  round  us  ;  their  regular  motions, 
which  are  explained  by  the  laws  of  attraction  on  the  first 
supposition,  would  he  unintelligible  on  the  last,  and  the  order 
and  harmony  of  the  universe  be  destroyed.  Think  what  an 
immense  circuit  the  sun  and  stars  would  make  daily,  were 
their  apparent  motions  real.  We  know  many  of  them  to  be 
bodies  more  considerable  than  our  earth  ;  for  our  eyes  vainly 
endeavour  to  persuade  us,  that  they  are  little  brilliants  spark- 
ling in  the  heavens,  while  science  teaches  us  that  they  are 
immense  spheres,  whose  apparent  dimensions  are  diminished 
by  distance.  Why  then  should  these  enormous  globes  daily 
traverse  such  a  prodigious  space,  merely  to  prevent  the 
necessity  of  our  earth's  revolving  on  its  axis  ? 


^J  ON  THE  PLANETS. 

Caroline.  I  think  I  must  now  be  convinced.  But  you 
wiilj  I  hope,  allow  me  a  little  time  to  familiarize  myself  to  an 
idea  so  different  from  that  which  I  have  been  accustomed  to 
entertain.     And  pray,  at  what  rate  do  we  move  ? 

Mrs.  B.  The  motion  produced  by  the  revolution  of  the 
earth  on  its  axis,  is  about  eleven  miles  a  minute,  to  an  inhab- 
itant of  London. 

Emily.  But  does  not  every  part  of  the  earth  move  with 
the  same  velocity  ? 

Mrs.  B.  A  moment's  reflection  would  convince  you  of 
the  contrary  ;  a  person  at  the  equator  must  move  quicker 
dian  one  situated  near  the  poles,  since  they  both  perform  a 
revolution  in  24  hours. 

Emily.  True,  the  equator  is  farthest  from  the  axis  of 
motion.  But  in  the  earth's  revolution  round  the  sun^  every 
part  must  move  with  equal  velocity  ? 

Mrs.  B.     Yes,  about  a  thousand  miles  a  mini'te. 

Caroline.  How  astonishing  ! — and  that  it  should  be  pos- 
sible for  us  to  be  insensible  of  such  a  rapid  motion.  You 
would  not  tell  me  this  sooner,  Mrs.  B.,  for  fear  f  increasing 
my  incredulity. 

Before  the  time  of  Newton,  was  not  the  earth  supposed  to 
be  in  the  centre  of  the  system,  and  the  sun,  moon,  and  stars  to 
revolve  round  it  ? 

Mrs.  B.  This  was  the  system  of  Ptolemy  in  ancient 
times  ;  but  as  long  ago  as  the  beginning  of  the  sixteenth  cen- 
tury it  was  discarded,  and  the  solar  system,  such  as  I  have 
shown  you,  was  established  by  the  celebrated  astronomer 
Copernicus,  and  is  hence  called  the  Copernican  system. 
But  the  theory  of  gravitation,  the  source  from  which  this 
beautiful  and  harmonious  arrangement  flows,  we  owe  to  the 
powerful  genius  of  Newton,  who  lived  at  a  much  later  period. 

Emily.  It  appears,  indeed,  far  less  difficult  to  trace  by 
observation  the  motion  of  the  planets,  than  to  divine  by  what 
power  they  are  impelled  and  guided.  I  wonder  how  the 
idea  of  gravitation  could  first  have  occurred  to  Sir  Isaac 
Newton  ? 

Mrs.  B.  It  is  said  to  have  been  occasioned  by  a  circum- 
stance from  which  one  should  little  have  expected  so  grand  a 
theory  to  have  arisen. 

During  the  prevalence  of  the  plague  in  the  year  l665, 
Newton  retired  into  the  country  io  avoid  the  contagion  :  when 


ON  THE  PLANETS.  97 

^^ikig  one  day  in  his  orchard  he  observed  an  apple  fail  from 

tree,  and  was  led  to  consider  what  could  be  the  cause  which 
jrought  it  to  the  ground. 

Caroline,  If  I  dared  to  confess  it,  Mrs.  B.,  I  should  say 
that  such  an  enquiry  indicated  rather  a  deficiency  than  a. 
superiority  of  intellect.  I  do  not  understand  how  any  one 
can  wonder  ai  what  is  so  natural  and  so  common. 

Mr^.  B.  It  is  the  mark  of  superior  genius  to  find  matter 
for  wonder,  observation,  and  research,  in  circumstances  which, 
to  the  ordinary  mind,  appear  trivial,  because  they  are  com- 
mon, and  with  which  they  are  satisfied,  because  they  are 
natural,  without  reflecting  that  nature  is  our  grand  field  of 
observation,  that  within  it  is  contained  our  whole  store  of 
knowledge  ;  in  a  word,  that  to  study  the  works  of  nature,  is 
to  learn  to  appreciate  and  admire  the  wisdom  of  God.  Thus, 
it  was  the  simple  circumstan€e  of  the  fall  of  an  apple,  which 
led  to  the  discovery  of  the  laws  upon  which  the  Copernicaii 
system  is  founded  ;  and  whatever  credit  this  system  had  ob- 
tained before,  it  now  rests  upon  a  basis  from  which  it  cannot 
be  shaken. 

Emily.  This  was  a  most  fortunate  apple,  and  more  wor- 
t^.y  to  be  commemorated  than  all  those  that  have  been  sung 
by  the  poets.  The  apple  of  discord  for  which  the  goddesses 
contended  ;  the  golden  apples  by  which  Atalanta  won  the 
race  ;  nay,  even  the  apple  which  William  Tell  shot  from  the 
head  of  his  son,  cannot  be  compared  to  this  ! 

The  Strx.  The  sun  is  a  spherical  body,  situated  near  tlie  centre  of 
gravity  in  the  system  of  planets,  of  which  our  earth  is  one.  Its  diameter 
IS  877,547  English  miles  ;  or  equal  to  100  diameters  of  the  earth  ;  and 
therefore  its  cubic  magnitude  must  exceed  that  of  the  earth  one  millioa 
of  times.  It  revolves  round  its  axis  in  25  days,  and  10  hours,  vhich  has 
been  determined  by  means  of  several  dark  spots  seen  with  telescopes  on 
that  luminary.  Dr.  Herschel  supposes  these  spots  in  the  sun  to  be  the 
appearance  of  the  opaque  body  of  the  sun  througl^  the  openings  in  his 
luminous  atapoaphere,. 


CONVERSATION  VIH- 


ON  THE  EARTH. 

Of  the  Terrestrial  Globe  ^  Of  the  Figure  of  the  Earth  ;  Of 
the  Pendulum  ;  Of  the  Variation  of  the  Seasons^  and  of 
the  Length  of  Days  and  Nights  ;  Of  the  causes  of  the 
Heat  of  Summer  ;  Of  Salary  Siderial^  and  Equal  or  Mean 
Time. 


MRS.  B. 

As  the  earth  is  the  planet  in  which  we  are  the  most  partic- 
ularly interested,  it  is  my  intention  this  morning,  to  explain  to 
you  the  effects  resulting  from  its  annual  and  diurnal  motions  ; 
but  for  this  purpose  it  will  be  necessary  to  make  j^ou  acquaint- 
ed with  the  terrestrial  globe  :  you  have  not  either  of  you^  I 
conclude,  learnt  the  use  of  the  globes  ?* 

Caroline.  No ;  I  once  indeed  learnt  by  heart  the  names 
of  the  lines  marked  on  the  globe,  but  as  I  was  informed  they 
were  only  im,aginary  divisions,  they  did  not  appear  to  me 
worthy  of  much  attention,  and  were  soon  forgotten. 

Mrs*  B»  You  suppose,  then,  that  astronomers  had  been 
at  the  trouble  of  inventing  a  number  of  lines  to  little  purpose. 
It  will  be  impossible  for  me  to  explain  to  you  the  particular 
effects  of  the  earth's  motion  without  your  having  acquired  a 
knowledge  of  these  lines  :  in  plate  VIH.  fig.  2.  you  will  find 

•  The  earth  is  of  a  globular  form.  For,  1.  The  shadow  of  the  earth 
projected  on  the  moon  in  an  eclipse  is  always  circular ;  which  appearance 
could  only  be  produced  by  a  spherical  body.  2.  The  convexity  of  the 
surface  of  the  sea  is  evident ;  the  mast  of  an  approaching  ship  being  seea 
before  its  hull.  3.  The  north  pole  becomes  more  elevated  by  travelling 
northward,  in  proportion  to  the  space  passed  over.  4.  Navigators  have 
sailed  round  the  earth,  and  by  steering  their  course  continually  weftward^ 
t^rriyed,  at  leogth^  at  the  plate  from  wheaoe  they  departed* 


ON  THE  JEARTH.  99 

them  all  delineated ;  and  you  must  learn  them  perfectly  if 
you  wish  to  make  any  proficiency  in  astronomy. 

Caroline,  I  was  taught  them  at  so  early  an  age  that  I 
could  not  understand  their  meaning  ;  and  I  have  often  heard 
you  say  that  the  only  use  of  words  was  to  convey  ideas. 

Mrs.  B,  The  names  of  these  lines  would  have  conveyed 
ideas  of  the  figures  they  were  designed  to  express,  though  the 
use  of  these  figures  might  at  that  time  have  been  too  difficult 
for  you  to  understand.  Childhood  is  the  season  when  impres- 
sions on  the  memory  are  most  strongly  and  most  easily  made  : 
k  is  the  period  at  which  a  large  stock  of  ideas  should  be  treas- 
ured up  J  the  application  of  which  we  may  learn  when  the 
understanding  is  more  developed.  It  is,  I  think,  a  very  mis- 
taken notion  that  children  should  be  taught  such  things  only^ 
as  they  can  perfectly  understand.  Had  you  been  early  made 
acquainted  with  the  terms  which  relate  to  figure  and  motion^ 
how  much  it  would  have  facilitated  your  progiess  in  natural 
philosophy.  I  have  been  obliged  to  confine  myself  to  the 
most  common  and  familiar  expressions,  in  explaining  the  laws 
of  nature,  though  1  am  convinced  that  appropriate  and  scien- 
tific terms  would  have  conveyed  more  precise  and  accurate 
ideas  ;  but  I  was  afraid  of  not  being  understood. 

Emily.  You  may  depend  upon  our  learning  the  names  of 
these  lines  thoroughly,  Mrs*  B.  ;  but  before  we  commit  them 
to  memory,  will  you  have  the  goodness  to  explain  them  to  us  ? 

Mrs.  B,  Most  willingly.  This  globe,  or  sphere,  repre- 
sents the  earth  ;  the  line  which  passes  through  its  centre,  and 
on  which  it  turns,  is  called  its  axis,  and  the  two  extremities  oi 
the  axis  A  and  B,  are  the  poles,  distinguished  by  the  names 
of  the  north  and  south  pole.  The  circle  C  D,  which  divides 
the  globe  into  two  equal  parts  between  the  poles,  is  called  the 
equator,  or  equinoctial  line  ;  that  part  of  the  globe  to  the 
north  of  the  equator  is  the  northern  hemisphere  ;  that  part 
to  the  south  of  the  equator,  the  southern  hemisphere.  The 
small  circle  E  F,  which  suj-rounds  the  north  pole,  is  called 
the  arctic  circle  ;  that  G  H,  which  surrounds  the  south  pole, 
the  antarctic  circle.  There  are  two  intermediate  circles  be- 
tween the  polar  circles  and  the  equator  ;  that  to  the  north, 
I  Kj  called  the  tropic  of  Cancer  ;  that  to  the  south,  L  M, 
called  the  tropic  of  Capricorn.  Lastly,  this  circle,  L  K, 
which  divides  the  globe  into  two  equal  parts,  crossing  the 
equator  and  extending  northward  as  far  as  the  tropic  of  Can- 
cer, and  southward  as  far  as  the  tropic  of  Capricorn,  is  called 


100  ON  THE  EARTH. 

tlie  ecliptic.  The  delineation  of  the  ecliptic  on  the  terrestrial 
globe  is  not  without  danger  of  conveying  false  ideas  ;  for  the 
ecliptic  (as  I  have  before  said)  is  an  imaginary  circle  in  the 
heavens  passing  through  the  middle  of  the  zodiac,  and  situated 
in  the  plane  of  the  earth's  orbit. 

Cai'oline,  I  do  not  understand  the  meaning  of  the  plane 
of  the  earth's  orbit. 

Mrs,  B.  A  plane,  or  plain,  is  an  even  level  surface.  Let 
us  suppose  a  smooth  thin  solid  plane  cutting  the  sun  through 
the  centre,  extending  out  as  far  as  the  fixed  stars,  and  termin- 
ating in  a  circle  which  passes  through  the  middle  of  the  zodiac  ; 
in  this  plane  the  earth  would  move  in  its  revolution  round  the 
sun  ;  it  is  therefore  called  the  plane  of  the  earth's  orbit,  and 
the  circle  in  which  this  plane  cuts  the  signs  of  the  zodiac  is 
the  ecliptic.  Let  the  fig,  1»  plate  IX.  represent  such  a  plane, 
S  the  sun,  E  the  earth  with  its  orbit,  and  A  B  C  D  the  echp- 
tic  passing  through  the  middle  of  the  zodiac. 

Emily,  li  tlie  ecliptic  relates  only  to  the  heavens,  why  is 
it  described  upon  the  terrestrial  globe  ? 

Mrs,  B,  It  is.  convenient  for  the  demonstration  of  a  vari- 
ety of  problems  in  the  use  of  the  globes  ;  and  besides,  the 
obliquity  of  this  circle  to  the  equator  is  rendered  more  con- 
spicuous by  its  being  described  on  the  same  globe  ;  and  the 
obliquity  of  the  ecliptic  shows  the  inclination  of  the  earth's 
axis  to  the  plane  of  its  orbit.  But  to  return  to  fig.  2.  plate 
VIII. 

The  spaces  between  the  several  parallel  circles  on  the  ter- 
restrial globe  are  called  zones  ;  that  which  is  comprehended 
between  the  tropics  is  distinguished  by  the  name  of  the  torrid 
zone  ;  the  spaces  which  extend  from  the  tropics  to  the  polar 
circles,  the  north  and  south  temperate  zones  ;  and  the  spaces, 
contained  within  the  polar  circles,  the  frigid  zones. 

The  several  lines  which,  you  observe,  are  drawn  from  one 
pole  to  the  other,  cutting  the  equator  at  right  angles,  are  called 
meridians.  When  EJjy  one  of  these  meridians  is  exactly 
opposite  the  sun  it  is  rkid-day,  or  twelve  o'clock  in  the  day, 
with  all  the  places  situated  on  that  meridian  ;  and,  with  the 
places  situated  on  the  opposite  meridian,  it  is  consequently 
midnight. 

Emihj,  To  places  situated  equally  distant  from  these  two 
meridians,  it  must  then  be  six  o'clock  ? 

Mrs.  B.     Yes :  if  thev  are  to  the  east  of  the  sun'$  meri- 


BLATJS  iX. 


ON  THE  EARTH.  lOi 

ciian  it  is  six  o'clock  in  the  afternoon,  because  the  sun  will 
have  previously  passed  over  them  ;  if  to  the  west,  it  is  six 
o'clock  in  the  morning,  and  the  sun  will  be  proceeding  to-^ 
wards  that  meridian. 

Those  circles  which  divide  the  globe  into  two  equal  parts^ 
such  as  the  equator  and  the  ecliptic,  are  called  greater  circles  ; 
to  distinguish  them  from  those  which  divide  it  into  two  une- 
qual parts,  as  the  tropics  and  polar  circles,  which  are  called 
lesser  circles.  All  circles  are  divided  into  360  equal  parts, 
called  degrees,  and  degrees  into  60  equal  parts,  called  minutes. 
The  diameter  of  a  circle  is  a  right  line  drawn  across  it^  and 
passing  through  the  centre  ;  for  instance,  the  boundary  of 
this  sphere  is  a  circle,  and  its  axis  the  diameter  of  that  circle  ; 
the  diameter  is  equal  to  a  little  less  than  one-third  of  the 
circumference.  Can  you  tell  me  nearly  how  many  degrees  it 
contains  ? 

Caroline,  It  must  be  something  less  than  one-third  of 
360  degrees,  or  nearly  120  degrees. 

Mrs,  B,  Right  ;  now  Emily  you  may  tell  me  exactly 
how  many  degrees  are  contained  in  a  meridian  ? 

Emily.  A  meridian,  reaching  from  one  pole  to  the  other, 
is  half  a  circle,  and  must  therefore  contain   180  degrees. 

Mrs,  B,  Very  well  ;  and  what  number  of  degrees  are 
there  from  the  equator  to  the  poles  ? 

Caroline,  The  equator  being  equally  distant  from  either 
pole,  that  distance  must  be  half  of  a  meridian,  or  a  quarter  of 
the  circumference  of  a  circle,  and  contain  90  degrees. 

Mrs,  B,  Besides  the  usual  division  of  circles  into  degrees^ 
the  ecliptic  is  divided  into  twelve  equal  parts,  called  signs, 
which  bear  the  names  of  the  constellations  through  which  this 
circle  passes  in  the  heavens.  The  degrees  measured  on  the 
meridians  from  north  to  south,  or  south  to  north,  are  called 
degrees  of  latitude  ;  those  measured  from  east  to  west  on  the 
equator,  the  echptic,  or  any  of  the  lesser  circles  are  called 
degrees  of  longitude  ;  hence  these  circles  bear  the  name  of 
longitudinal  circles  ;  they  are  also  called  parallels  of  latitude. 

Emily,  The  degrees  of  longitude  must  then  vary  in  length 
according  to  the  dimensions  of  the  circle  on  which  they  are 
reckoned ;  those^  for  instance,  at  the  polar  circles  will  be 
considerably  smaller  than  those  at  the  equator  ? 

Mrs,  B,     Certainly ;  since  the  degrees  of  circles  of  differ- 
ent dimensions  do  not  vary  in  number,  they  must  necessarily 
9# 


I0::i  ON  THE  EARTB. 

vary  in  length.  The  degrees  of  latitude,  3^011  may  observe^ 
never  vary  m  length  ;  for  the  meridians  on  which  they  axe 
reckoned  are  all  of  the  same  dimensions. 

Emily.     And  of  what  length  is  a  degree  of  latitude  ? 

Mrs,  B,  Sixty  geographical  miles,  which  is  equal  to  69  i 
English  statute  miles. 

Emily,  The  degrees  of  longitude  at  the  equator  must  then 
be  of  the  same  dimensions  ? 

Mrs,  B,  They  would,  were  the  earth  a  perfect  sphere  ; 
but  its  form  is  not  exactly*  spherical,  being  somewhat  protu- 
berant about  the  equator,  and  flattened  towards  the  poles. 
This  form  is  supposed  to  proceed  from  the  superior  action  of 
the  centrifugal  power  at  the  equator. 

Caroline,  I  thought  I  had  understood  the  centrifugal  force 
perfectly,  but  I  do  not  comprehend  its  effect  in  this  instance. 

Mrs.  B,  You  know  that  the  revolution  of  the  earth  on  its 
axis  must  give  every  particle  a  tendency  to  fly  ofl*  from  the 
centre,  that  this  tendency  is  stronger  or  weaker  in  proportion 
to  the  velocity  with  which  the  y  article  moves  ;  now  a  parti- 
cle situated  near  one  of  the  polar  circles  makes  one  rotation 
in  the  same  space  of  time  as  a  particle  at  the  equator  ;  the 
latter,  therefore,  having  a  much  larger  circle  to  describe^ 
travels  proportionally  faster,  consequently  the  centrifugal 
force  is  much  stronger  at  the  equator  than  at  the  polar  circles  : 
it  gi'adually  decreases  as  you  leave  the  equator  and  approach 
the  poles,  where,  as  there  is  no  rotatory  motion,  it  entirely 
ceases.  Supposing,  therefore,  the  earth  to  have  been  origin- 
ally in  a  fluid  state,  the  particles  in  the  torrid  zone  would 
recede  much  farther  from  the  centre  than  those  in  the  frigid 
zones  ;  thus  the  polar  regions  would  become  flattened,  and 
those  about  the  equator  elevated. 

Caroline.  I  did  not  consider  that  the  particles  in  the 
neighborhood  of  the  equator  move  with  greater  velocity  than 
those  about  the  poles  ;  this  was  the  reason  I  could  not  under- 
stand you. 

Mrs,  B.  You  must  be  careful  to  remember,  that  those 
j^arts  of  a  body  which  are  farthest  from  the  centre  of  motion 
must  move  with  the  greatest  velocity  :  the  axis  of  the  earth 
is  the  centre  of  its  diurnal  motion,  and  the  equatorial  regions 
the  parts  most  distant  from  the  axis. 

Caroline.  My  head  then  moves  faster  than  my  feet ;  and 
upon  the  summit  of  a  mountain  we  are  carried  round  quicker 
^anin  a  vallev? 


Mrs*  B.  Certainlyj  your  head  is  more  distant  from  the 
centre  of  motion,  than  your  feet;  the  mountain-top  than  the 
valley  :  and  the  more  distant  any  part  of  a  body  is  from  the 
centre  of  motion,  the  larger  is  the  circle  it  will  describe,  and 
the  greater  therefore  must  be  its  velocity. 

Emily,  I  have  been  reflecting  that  if  the  earth  is  not  a 
perfect  circle..... 

Mrs,  B.  A  sphere  you  mean,  my  dear  ;  a  circle  is  a 
round  line,  every  part  of  which  is  equally  distant  from  the 
centre  ;  a  sphere  or  globe  is  a  round  body^  the  surface  of 
which  is  every  where  equally  distant  from  the  centre. 

Emily.  If,  then,  the  earth  is  not  a  perfect  sphere,  but 
prominent  at  the  equator,  and  depressed  at  the  poles,  would 
not  a  body  weigh  heavier  at  the  equator  than  at  the  poles  ? 
For  the  earth  being  thicker  at  the  equator,  the  attraction  of 
gravity  perpendicularly  downwards  must  be  stronger. 

Mrs,  B.  Your  reasoning  has  some  plausibility,  but  I  am 
sorry  to  be  obliged  to  add  that  it  is  quite  erroneous  ;  for  the 
nearer  any  part  of  the  surface  of  a  body  is  to  the  centre  of 
attraction,  the  more  strongly  it  is  attracted  ;  because  the  most 
considerable  quantity  of  matter  is  about  that  centre.  In  re- 
gard to  its  effects,  you  might  consider  the  power  of  gravity,  as 
that  of  a  magnet  placed  at  the  centre  of  attraction. 

Emily,  But  were  you  to  penetrate  deep  into  the  earth, 
would  gravity  increase  as  you  approached  the  centre  ? 

Mrs,  B,  Certainly  not  ;  I  am  referring  only  to  any  sit- 
uation on  the  surface  of  the  earth.  Were  you  to  penetrate 
into  the  interior,  the  attraction  of  the  parts  above  you  would 
counteract  that  of  the  parts  beneath  you,  and  consequently 
diminish  the  power  of  gravity  in  proportion  as  you  approached 
the  centre  ;  and  if  you  reached  that  point,  being  equally 
attracted  by  the  parts  all  around  you,  gravity  would  cease,  and 
you  would  be  without  weight. 

Emily,  Bodies  then  should  weigh  less  at  the  equator  than 
at  the  poles,  since  they  are  more  distant  from  the  centre  of 
gravity  in  the  former  than  in  the  latter  situation. 

Mrs^  B,  And  this  is  really  the  case  ;  but  the  difference 
of  weight  would  be  scarcely  sensible,  were  it  not  augmented  by 
another  circumstance. 

Caroline,  And  what  is  this  singular  circumstance  which 
seems  to  disturb  the  laws  of  nature  ? 

Mrs.  B,  One  that  you  are  well  acquainted  with,  as  con- 
^^ucing  more  to  the  preservation  than  the  destruction  of  order, 


104  ON  THE  EARTH, 

— ^the  centrifugal  force.  This  we  have  just  observed  to  be 
stronger  ai  the  equator  ;  and  as  it  tends  to  drive  bodies  from 
the  centre,  it  is  necessarily  opposed  to,  and  must  lessen  the 
power  of  gravity?  which  attracts  them  towards  the  centre. 
We  accordingly  find  that  bodies  weigh  lightest  at  the  equator, 
ivhere  the  centrifugal  force  is  greaiest ;  and  heaviest  at  the 
poles,  where  this  power  is  least.* 

Caroline.  Has  the  experiment  been  made  in  these  differ- 
ent situations  ? 

Mrs.  B,  Lewis  XIV.,  of  France,  sent  philosophers  both 
to  the  equator  and  to  Lapland  for  this  purpose  ;  the  severity 
of  the  climate,  and  obstruction  of  the  ice,  has  hitherto  rendered 
every  attempt  to  reach  the  pole  abortive  ;  but  the  difference 
of  gravity  at  the  equator  and  in  Lapland  is  very  perceptible. 
Caroline.  Yet  I  do  not  comprehend,  how  the  difference 
of  weight  could  be  ascertained  ;  for  if  the  body  under  trial 
decreased  in  weight,  the  weight  which  was  opposed  to  it  in 
the  opposite  scale  must  have  diminished  in  the  same  propor- 
tion. For  instance,  if  a  pound  of  sugar  did  not  weigh  so  heavy 
at  the  equator  as  at  the  poles,  the  leaden  pound  which  served 
to  weigh  it,  would  not  be  so  heavy  either  :  therefore  they 
would  still  balance  each  other,  and  the  different  force  of 
gravity  could  not  be  ascertained  by  this  means. 

Mrs.  B.  Your  observation  is  perfectly  just :  the  drfference 
of  gravity  of  bodies  situated  at  the  poles  and  at  the  equator 
cannot  be  ascertained  by  weighing  them  ;  a  pendulum  was 
therefore  used  for  that  purpose. 

Caroline.  What,  the  pendulum  of  a  clock  ?  how  could 
that  answer  the  purpose  ? 

Mrs.  B.  A  pendulum  consists  of  a  line,  or  rod,  to  one  end 
of  which  a  weight  is  attached,  and  it  is  suspended  by  ihe 
other  to  a  fixed  point,  about  which  it  is  made  to  vibrate. 
Without  being  put  in  motion,  a  pendulum,  like  a  plumb  line, 
hangs  perpendicular  to  the  genera!  surface  of  the  earth,  by 
which  it  is  attracted  ;  but,  if  you  raise  a  pendulum,  gravity 
will  bring  it  back  to  its  perpendicular  position.  It  will,  how- 
ever, not  remain  stationary  there,  for  the  velocity  it  has 
received  during  its  descent  will  impel  it  onwards,  and  it  will 
rise  on  the  opposite  side  to  an  equal  height  :  from  thence  it 

*  If  the  diurnal  raotion  of  the  earth  round  its  axis  was  about  17  times 
faster  than  it  is,  th  centrifugal  force  would,  at  the  equator,  be  equal  to 
the  power  of  gravity,  and  all  Hodi^s  there  would  entirely  lose  weight 
Bat  if  the  earth  revolved  still  quicker  thau  this,  they  would  all  fly  off. 


ON  THE  EARTH.  10? 

is  brought  back  by  gravity,  and  again  driven  by  the  impulse 
of  its  velocity. 

Caroline.  If  so,  the  motion  of  a  pendulum  would  be  per- 
petual, and  I  thought  you  said  that  there  was  no  perpetual 
motion  on  the  earth. 

Mrs,  B,  The  motion  of  a  pendulum  is  opposed  by  the 
resistance  of  the  air  in  which  it  vibrates,  and  by  the  friction 
of  the  part  by  which  it  is  suspended  :  were  it  possible  to  re- 
move these  obstacles,  the  motion  of  a  pendulum  would  be 
perpetual,  and  its  vibrations  perfectly  regular  :  being  of  equal 
distances,  and  performed  in  equal  times.* 

Emily,  That  is  the  natural  result  of  the  uniformity  of  the 
power  which  produces  these  vibrations,  for  the  force  of  gravity 
being  always  the  same,  the  velocity  of  the  pendulum  must 
consequently  be  uniform. 

Caroline.  No,  Emily,  you  are  mistaken  ;  the  cause  is  not 
always  uniform,  and  therefore  the  effect  will  not  be  so  either, 
I  have  discovered  it,  Mrs.  B.  ;  since  the  force  of  gravity  is 
less  at  the  equator  than  at  the  poles,  the  vibrations  of  the 
pendulum  will  be  slower  at  the  equator  than  at  the  poles. 

Mrs,  B,  You  are  perfectly  right,  Caroline ;  it  was  by  this 
means  that  the  difference  of  gravity  was  discovered,  and  the 
true  figure  of  the  earth  ascertained. 

Emily,  But  how  do  they  contrive  to  regulate  their  time  in 
the  equatorial  and  polar  regions  ?  for,  since  in  this  part  of  the 
earth  the  pendulum  of  a  clock  vibrates  exactly  once  in  a 
second,  if  it  vibrates  faster  at  the  poles  and  slower  at  the 
equator,  the  inhabitants  must  regulate  their  clocks  in  a  differ- 
ent manner  from  ours. 

Mrs,  B,  The  only  alteration  required  is  to  lengthen  the 
pendulum  in  one  case,  and  to  shorten  it  in  the  other  ;  for  the 
velocity  of  the  vibrations  of  a  pendulum  depends  on  its  length ; 
and  when  it  is  said,  that  a  pendulum  vibrates  quicker  at  the 
pole  than  at  the  equator,  it  is  supposing  it  to  be  of  the  same 
length.     A  pendulum  which  vibrates  a  second  in  this  latitude 

*  The  vibrations  ot  pendulums  are  subject  to  many  irregularities,  for 
which  no  effectual  remedy  has  yet  been  devised.  These  are  owing-  partly 
to  the  variable  density  and  temperature  of  the  air,  partly  to  the  rigidity 
and  friction  of  the  rod  by  which  they  are  suspended,  and  principally  to 
the  dilatation  and  contraction  of  the  materials,  of  which  they  are  formed. 
The  metalline  rods  of  pendulums  are  expanded  by  heat,  and  contracted 
by  cold  ;  therefore  clocks  will  go  faster  m  winter^  and  slower  in  summer. 
The  common  remedy  for  this  inconvenience  is  the  raising  or  lowering  th© 
bob  ©f  the  peadulum,  by  raieaDS  of  a  screwj  as  occasion  may  re<jujre. 


106  ON  THE  EARTH. 

is  064  inches  long.  In  order  to  vibrate  at  the  equator  in  the 
same  space  of  time,  it  must  be  lengthened  by  the  addition 
of  a  few  lines  ;  and  at  the  poles,  it  must  be  proportionally 
shortened.* 

I  shall  now,  I  think,  be  able  to  explain  to  you  the  variation 
of  the  seasons,  and  the  differefice  of  the  lengih  of  the  days  and 
nights  in  those  seasons  ;  both  effects  resulting  from  the  same 
cause. 

In  moving  round  the  sun,  the  axis  of  ihe  earth  is  not  per- 
pendicular to  the  plane  of  its  orbit.  Supposhig  tliis  round 
table  to  represent  the  plane  of  the  earth^'s  orbit,  and  this  litde 
globe,  which  has  a  wire  passing  through  it,  representing  the 
axis  and  poles,  we  shall  call  the  earth  ;  in  moving  round  the 
table,  the  wire  is  not  perpendicular  to  it,  but  oblique. 

Emily,  Yes,  I  understand  the  earth  does  not  go  round  the 
sun  in  an  upriglit  position,  its  axis  is  slanting  or  oblique. 

Mrs,  S.  All  the  lines,  which  you  learnt  in  your  last  lesson, 
are  delineated  on  this  little  globe  ;  you  must  consider  the 
ecliptic  as  representing  the  plane  of  the  earth's  orbit ;  and  the 
equator  which  crosses  the  ecliptic  in  two  places,  shows  the 
degree  of  obliquity  of  the  axis  of  the  earth  in  that  orbit,  which 
is  exactly  234  degrees.  The  points  in  which  the  ecliptic 
intersects  the  equator  are  called  nodes. 

Bui  I  believe  I  shall  make  this  clear  to  you  by  revolving 
the  little  globe  round  a  candle,  w^hich  shall  represent  the  sun. 
(Plate  IX:  fig.  2.) 

As  I  now^  hold  it,  at  A,  you  see  it  in  the  situation  in  which 
it  is  in  the  midsi  of  summer,  or  what  is  called  the  summer 
solstice,  which  is  on  the  21st  of  June. 

Emily,  You  hold  the  wire  awry,  I  suppose,  in  order  to 
show  that  the  axis  of  the  earth  is  not  upright  ? 

Mrs,  B,  Yes  ;  in  summer,  the  north  pole  is  inclined 
towards  the  sun.  In  this  season,  therefore,  the  northern 
hemisphere  enjoys  much  more  of  his  rays  than  the  southern. 
The  sun,  you  see,  now  shines  over  the  whole  of  the  north 

*  What  is  here  stated  concerning  the  length  of  pendulums  as  connected 
with  the  force  of  gravity  is  at  complete  variance  with  fact.  The  force  of 
gravitation  is  greater,  it  is  well  knowB,  at  the  poles  than  at  the  equator  ; 
and  since  the  vibration  of  pendulums  is  occasioned  by  gravity,  the  lengths 
of  pendulums  vibrating  in  the  same  time  must  evidently  be  proportioned 
to  the  gravities  at  the  places  v/here  they  vibrate.  Accoi-dingly,  it  is  found^^ 
by  observation,  in  order  to  vibrate,  at  the  equator,  in  the  same  space,  the 
pendulum  must  not  be  lengthened,  as  above  stated,  but  shortened  ;  aiad, 
at  the  poles,  it  must  not  be  shortened,  but  proportionally  ieDgthened. 


ON  THS  EAETH,  107 

frigid  zone^  and  notwithstanding  the  earth's  diurnal  revolution, 
which  I  imitate  by  twirling  the  ball  on  the  wire,  it  will  con- 
tinue to  shine  upon  it  as  long  as  it  remains  in  this  situation, 
whilst  the  south  frigid  zone  is  at  the  same  thne  completely  in 
obscurity. 

Caroline.  That  is  very  strange :  I  never  before  heard 
that  there  was  constant  day  or  night  in  any  part  of  the  world ! 
How  much  happier  the  inhabitants  of  the  north  frigid  zone 
must  be  than  those  of  the  southern  ;  the  first  enjoy  uninter- 
rupted day,  while  the  last  are  involved  in  perpetual  darkness. 

Mrs,  B.  You  judge  with  too  much  precipitation ;  examine 
a  little  further,  and  you  will  find,  that  the  two  frigid  zones 
share  an  equal  fate. 

We  shall  now  make  the  earth  set  off  from  its  position  in  the 
summer  solstice,  and  carry  it  round  the  sun ;  observe  th^t  the 
pole  is  always  inclined  in  the  same  direction,  and  points  to  the 
same  spot  in  the  heavens.  There  is  a  fixed  star  situated  near 
that  spot,  which  is  hence  called  the  North  Polar  star.  Now 
let  us  stop  the  earth  at  B,  and  examine  it  in  its  present  situa- 
tion ;  it  has  gone  through  one  quarter  of  its  orbit,  and  is  arrived 
at  that  point  at  which  the  ecliptic  cuts  or  crosses  the  equator, 
and  which  is  called  the  autumnal  equinox. 

Emily.     That  is  then  one  of  the  nodes. 

The  sun  now  shines  from  one  pole  to  the  other,  just  as  it 
would  constantly  do,  if  the  axis  of  the  earth  were  perpendicu- 
lar  to  its  orbit. 

Mrs,  B.  Because  the  inclination  of  the  axis  is  now  neither 
towards  the  sun  nor  in  the  contrary  direction  ;  at  this  period 
of  the  year,  therefore,  the  days  and  nights  are  equal  in  every 
part  of  the  earth.  But  the  next  step  she  takes  in  her  orbit, 
you  see,  involves  the  north  pole  in  darkness,  whilst  it  illumines 
that  of  the  south  ;  this  change  was  gradually  preparing  as  I 
moved  the  earth  from  summer  to  autumn  ;  the  arctic  circle, 
which  was  at  first  entirely  illumined,  began  to  have  short 
nights  J  which  increased  as  the  earth  approached  the  autumnal 
equinox  ;  and  the  instant  it  passed  that  point,  the  long  night 
of  the  north  pole  commences,  and  the  south  pole  begins  to 
enjoy  the  light  of  the  sun.  We  shall  now  make  the  earth 
proceed  in  its  orbit,  and  you  may  observe  that  as  it  advances, 
the  days  shorten,  and  the  nights  lengthen,  throughout  the 
northern  hemisphere,  until  it  arrives  at  ^e  winter  solstice,  on 
the  21st  of  December,  when  the  north  frigid  zone  is  entirely 
in  darkness,  and  the  southern  has  uninterrupted  day-light* 


108  ON  THE  EARTH. 

iJaroUne.  Then  after  all,  the  sun  which  I  thought  so  par- 
tial, confers  his  favors  equally  on  all. 

Mrs.  B.  Not  so  neither :  the  inhabitants  of  the  torrid 
zone  have  much  more  heat  than  we  have,  as  the  sun's  rays 
fall  perpendicularly  on  them,  while  they  shine  obliquely  on 
the  rest  of  the  world,  and  almost  horizontally  on  the  poles  ; 
for  during  their  long  day  of  six  months,  the  sun  moves  round 
their  horizon  without  either  rising  or  setting  ;  the  only  obser- 
vable difference,  is,  that  it  is  more  elevated  by  a  few  degrees 
at  mid-day,  than  at  mid-night. 

Emily,  To  a  person  placed  in  the  temperate  zone,  in  the 
situation  in  which  we  are  in  England,  the  sun  will  shine 
neither  so  obliquely  as  it  does  on  the  poles,  nor  so  vertically 
as  at  the  equator  ^  but  its  rays  will  fall  upon  him  more 
obliquely  in  autumn  and  winter,  than  in  summer. 

Caroline.  And  therefore,  the  inhabitants  of  the  temperate 
zones,  will  not  have  merely  one  day  and  one  night  in  the  year 
as  happens  at  the  poles,  nor  will  they  have  equal  days  and 
equal  nights  as  at  the  equator ;  but  their  days  and  nights  will 
vary  in  length,  at  different  times  of  the  year,  according  as 
their  respective  poles  incline  towards  or  from  the  sun,  and  the 
difference  will  be  greater  in  proportion  to  their  distance  from 
the  equator. 

Mrs.  B.  We  shall  now  follow  the  earth  through  the  other 
lialf  of  her  orbit,  and  you  will  observe,  that  now  exactly  the 
same  effect  takes  place  in  the  southern  hemisphere,  as  what  we 
have  just  remarked  in  the  northern.  Day  commences  at  the 
south  pole  when  night  sets  in  at  the  north  pole  ;  and  in  every 
other  part  of  the  southern  hemisphere  the  days  are  longer 
than  the  nights,  white,  on  the  contrary,  our  nights  are  longer 
than  our  days.  When  the  earth  arrives  at  the  vernal  equinox, 
D,  where  the  ecliptic  again  cuts  the  equator,  on  the  25th  of 
March,  she  is  situated  with  respect  to  the  sun,  exactly  in  the 
same  position,  as  in  tiie  autumnal  equinox  ;  and  the  only 
difference  with  respect  to  the  earth,  is,  that  it  is  now  autumn 
in  the  southern  hemisphere,  whilst  it  is  spring  with  us. 

Caroline.  Then  the  days  and  nights  are  again  every  where 
equal  ? 

Mrs.  B.  Yes,  for  the  half  of  the  globe  which  is  enlightened, 
extends  exactly  from  one  pole  to  the  other,  the  day  breaks  to 
the  north  pole,  and  the  sun  sets  to  the  south  pole;  but  in  every 
other  part  of  the  globe,  the  day  and  night  is  of  twelve  hours 


0:S  THE  EAftTII.  109 

length  J  hence  the  word  equinox^  which  is  derived  from  the 
Latin,  meaning  equal  night. 

As  the  earth  proceeds  towards  summer,  the  days  lengthen 
in  the  northern  hemisphere,  and  shorten  in  the  southern,  till 
the  earth  reaches  the  summer  solstice,  when  the  north  frigid 
zone  is  entirely  illumined,  and  the  southern  is  in  complete 
darkness  ;  and  we  have  now  brought  the  earth  again  to  the 
spot  from  whence  we  first  accompanied  her. 

Emily,  This  is  indeed;  a  most  satisfactory  explanation  of 
the  seasons  ;  and  the  more  I  learn,  the  more  I  admire  the 
simplicity  of  means  by  which  such  wonderful  effects  are 
produced. 

Mrs,  B,  I  know  not  which  is  most  worthy  of  our  admira- 
tion, the  cause,  or  the  effect  of  the  earth's  revolution,  round 
the  sun.  The  mind  can  find  no  object  of  contemplation, 
more  sublime,  than  the  course  of  this  magnificent  globe, 
impelled  by  the  combined  powers  of  projection  and  attraction 
to  roll  in  one  invariable  course  around  the  source  of  light  and 
heat :  and  what  can  be  more  delightful  than  the  beneficent 
effects  of  this  vivifying  power  on  its  attendant  planet.  It  is  at 
once  the  grand  principle  which  animates  and  fecundates 
nature. 

Emily.  There  is  one  circumstance  in  which  this  little 
ivory  globe  appears  to  me  to  differ  from  the  earth  ;  it  is  not 
quite  dark  on  that  side  of  it,  which  is  turned  from  the  candle, 
as  is  the  case  with  the  earth  when  neither  moon  nor  stars  are 
visible. 

Mrs^  B.  This  is  owmg  to  the  light  of  the  candle  being 
reflected  by  the  walls  of  the  room  on  every  part  of  the  globe, 
consequently  that  side  of  the  globe  on  which  the  candle  does 
not  directly  shine,  is  not  in  total  darkness.  Now  the  skies 
have  no  walls  to  reflect  the  sun's  light  on  that  side  of  our 
earth  which  is  in  darkness. 

Caroline,  I  beg  your  pardon,  Mrs.  B.,  I  think  that  the 
moon  and  stars  answer  the  purpose  of  walls  in  reflecting  the 
sun's  light  to  us  in  the  night. 

Mrs,  B.  Very  well,  Caroline  ;  that  is  to  say,  the  moon 
and  planets  ;  for  the  fixed  stars,  you  know,  shine  by  their 
own  light. 

Emily,  You  say  that  the  superior  heat  of  the  equatorial 
parts  of  the  earth,  arises  from  the  rays  falling  perpendicularly 
on  those  regions,  whilst  they  fall  obliquely  on  these  more 

io 


ilO  ON  THE  EARTH. 

northern  regions  ;  now  I  do  not  understand  why  perpendicu- 
lar rays  should  aflbrd  more  heat  than  oblique  rays. 

Caroline.  You  need  only  hold  your  hand  perpendicularly 
over  the  candle,  and  then  hold  it  sideways  oWiquely,  to  be 
sensible  of  the  difference. 

Emily.  I  do  not  dowbt  the  fact,  but  I  wish  to  have  it 
explained. 

Mrs.  B.  You  are  quite  right ;  if  Caroline  had  HOt  been 
satisfied  with  ascertaining  the  fact,  without  understanding  it, 
she  would  not  have  brought  forward  the  candle  as  an  illustra- 
tion ;  the  reason  why  you  feel  so  much  more  heat  if  you  hold 
your  hand  perpendicularly  over  the  candle,  than  if  you  hold 
it  sideways,  is  because  a  stream  of  heated  vapour  constantly 
ascends  from  the  candle,  or  any  other  burning  body,  which 
being  lighter  than  the  air  of  the  room,  does  not  spread  laterally 
but  rises  perpendicularly,  and  this  led  you  to  suppose  that  the 
rays  were  hotter  in  the  latter  direction.  Had  you  reflected, 
you  would  have  discovered  that  rays  issuing  from  the  candle 
sideways,  are  no  less  perpendicular  to  your  hand  w^hen  held 
opposite  to  them,  than  the  rays  which  ascend  when  your  hand 
is  held  over  them. 

The  reason  why  the  sun's  rays  afford  less  heat  when  in  an 
oblique  direction  than  when  perpendicular,  is  because  fewer 
of  them  fall  upon  an  equal  portion  of  the  earth  ;  this  will  be 
understood  better  by  referring  to  plate  X.  fig.  1,  which  repre- 
sents two  equal  portions  of  the  sun's  rays,  shining  upon  differ- 
ent parts  of  the  earth.  Here  it  is  evident  that  the  same  quan- 
tity of  rays,  fall  on  the  space  A  B,  as  fall  on  the  space  B  C  ; 
and  as  A  B  is  less  than  B  C,  the  heat  and  light  will  be  much 
stronger  in  the  former  than  in  the  latter ;  A  B,  you  see, 
represents  the  equatorial  regions,  where  the  sun  shines  per- 
pendicularly ;  and  B  C,  the  temperate  and  frozen  climates, 
where  his  rays  fall  more  obliquely. 

Emily.  This  accounts  not  only  for  the  greater  heat  of  the 
equatorial  regions,  but  for  the  gi  eater  heat  of  summer  ;  as  the 
sun  shines  less  obliquely  in  summer  than  in  winter* 

Mrs.  B.  This  you  will  see  exemplified  in  figure  2,  in 
which  the  earth  is  represented,  as  it  is  situated  on  the  21st 
June,  and  England  receives  less  oblique  and  consequently  a 
greater  number  of  rays,  than  at  any  other  season  ;  and  figure 
3,  shows  the  situation  of  England  on  the  21st  December, 
when  the  rays  of  the  sun  fall  most  obliquely  upon  her.  But 
there  is  also  another  reason  why  oblique  rays  give  less  heat, 


PLATE    A 


Tij    4- 


^■■^A 


ON  THE  EARTH*  111 

tiian  perpendicular  rays  ;  which  is,  that  they  have  a  greater 
portion  of  the  atmosphere  to  traverse ;  and  though  it  is  true, 
that  the  atmosphere  is  itself  a  transparent  body,  freely  admit- 
ting the  passage  of  the  sun's  rays,  yet  it  is  always  loaded  more 
or  less  with  dense  and  foggy  vapor,  which  the  rays  of  the  sun 
cannot  easily  penetrate  ;  therefore  the  greater  the  quantity  of 
atmosphere  the  sun's  rays  have  to  pass  through  in  their  way 
to  the  earth,  the  less  heat  they  will  retain  when  they  reach  it. 
This  will  be  better  understood,  by  referring  to  figure  4.  The 
dotted  line  round  the  earth,  describes  the  extent  of  the  atmos- 
phere, and  the  lines  which  proceed  from  the  sun  to  the  earth, 
the  passage  of  two  equal  portions  of  the  sun's  rays  to  the 
equatorial  and  polar  regions  ;  the  latter,  you  see,  from  its 
greater  obliquity  passes  through  a  greater  extent  of  atmosphere. 

Caroline,  And  this,  no  doubt,^  is  the  reason  why  the  sun 
in  the  morning  and  the  evening  gives  so  much  less  heat,  than 
at  mid-day. 

Mrs.  B.  The  diminution  of  heat,  morning  and  evening,  is 
certainly  owing  to  the  greater  obliquity  of  the  sun's  rays  ;  and 
as  such  they  are  affected  by  both  the  causes,  which  I  have 
just  explained  to  you  ;  the  difficulty  of  passing  through  a 
foggy  atmosphere  is  perhaps  more  particularly  applicable  to 
them,  as  mists  and  vapors  are  very  prevalent  about  the  time 
of  sunrise  and  sunset.  But  the  diminished  obliquity  of  the 
sun's  rays,  is  not  the  sole  cause  of  the  heat  of  summer  ;  the 
length  of  the  da^^s  greatly  conduces  to  it  :  for  the  longer  the 
sun  is  above  the  horizon,  the  more  heat  he  will  communicate 
to  the  earth. 

Caroline,  Both  the  longest  days,  and  the  most  perpen- 
dicular rays,  are  on  the  21st  of  June  ;  and  yet  the  greatest, 
heat  prevails  in  July  and  August. 

Mrs,  B,  Those  parts  of  the  earth  which  are  once  heated, 
retain  the  heat  for  some  length  of  time,  and  the  additional 
heat  they  receive,  occasions  an  elevation  of  temperature, 
although  the  days  begin  to  shorten,  and  the  sun's  rays  fall 
more  obliquely.  For  the  same  reason,  we  have  generally 
more  heat  at  three  o'clock  in  the  afternoon,  than  at  twelve 
when  the  sun  is  on  the  meridian. 

Emily,  And  pray,  have  the  other  planets  the  same  vicis- 
situdes of  seasons,  as  the  earth  ? 

Mrs,  B,  Some  of  them  more,  some  less,  according  as  their 
axes  deviate  more  or  less  from  the  perpendicular  to  the  plane 
of  their  orbits.     The  axis  of  Jupiter  is  nearly  perpendicular 


112  ON  THE  £ARTH. 

to  the  plane  of  his  orbit  ;  the  axes  of  Mars  and  of  Saturn  are 
each  inchned  at  angles  of  about  sixty  degrees  ;  whilst  the  axis 
of  Venus  is  believed  to  be  elevated  only  fifteen  or  twenty 
degrees  above  her  orbit  ;  the  vicissitudes  of  her  seasons  must 
therefore  be  considerably  greater  than  ours.  For  further  par- 
ticulars respecting  the  planets^  I  shall  refer  you  to  Bonnycas- 
tle's  Introduction  to  Astronomy. 

I  have  but  one  more  observation  to  make  to  you  relative  to 
the  earth's  motion,  which  is,  that  although  we  have  but  365 
days  and  nights  in  the  year,  she  performs  366  complete 
revolutions  on  her  axis  during  that  time. 

Caroline*  How  is  that  possible  ?  for  every  complete 
revolution  must  bring  the  same  place  back  to  the  sun.  It  is 
now  just  twelve  o'clock,  the  sun  is,  therefore,  on  our  meridian  ; 
in  twenty-four  hours  will  it  not  he  returned  to  our  meridian 
again,  and  will  not  the  earth  have  made  a  complete  rotation 
on  its  axis. 

Mrs>  B,  If  the  earth  had  no  progressive  motion  in  its 
orbit  whilst  it  revolves  on  its  axis,  this  would  be  the  case  ;  but 
as  it  advances  almost  a  degree  westward  in  its  orbit,  in  the 
same  time  that  it  completes  a  revolution  eastward  on  its  axis, 
it  must  revolve  nearly  one  degree  more  in  order  to  bring  the 
same  meridian  back  to  the  sun. 

Caroline,  Oh,  yes  !  it  will  require  as  much  more  of  a 
second  revolution  to  bring  the  same  meridian  back  to  the  sun, 
as  is  equal  to  the  space  the  earth  has  advanced  in  her  orbit, 
that  is,  nearly  a  degree  ;  this  difference  is,  however,  very 
little. 

Mrs,  B,  These  small  daily  portions  of  rotation  are  each 
equal  to  the  three  hundred  and  sixty-fifth  part  of  a  circle, 
which  at  the  end  of  the  year  amounts  to  one  complete  rotation. 

Emily,  That  is  extremely  curious.  If  the  earth,  then, 
had  no  other  than  its  diurnal  motion,  we  should  have  S6G 
days  in  the  year. 

Mrs,  B,  We  should  have  366  days  in  the  same  period  of 
time  that  we  now  have  S6^  ;  but  if  we  did  not  revolve  round 
the  sun,  we  should  have  no  natural  means  of  computing  years, 

You  will  be  surprised  to  hear,  that  if  time  is  calculated  by 
the  stars  instead  of  the  sun,  the  irregularity  which  we  have 
just  noticed  does  not  occur,  and  that  one  complete  rotation  of 
the  earth  on  its  axis,  brings  the  same  meridian  back  to  any 
fixed  star. 

fmihj.     That  seems  quite  unaccountable  :  for  the  eai'th 


ON  THE  EARTH.  IW 

advances  in  her  orbit  with  regard  to  the  fixed  stars,  the  same 
as  with  regard  to  the  sun. 

Mrs,  B,  True  J  but  then  the  distance  of  the  fixed  stars  is 
so  immensej  that  our  solar  system  is  in  comparison  to  it  but  a 
spot,  and  the  whole  extent  of  the  earth's  orbit  but  a  point  ; 
therefore,  whether  the  earth  remained  stationary,  or  whether 
it  revolved  in  its  orbit  during  its  rotation  on  its  axis,  no  sensible 
difference  would  be  produced  with  regard  to  the  fixed  stars. 
One  complete  revolution  brings  the  same  meridian  back  to  the 
same  fixed  star  ;  hence  the  fixed  stars  appear  to  go  round  the 
earth  in  a  shorter  time  than  the  sun  by  three  minutes  fifty-six 
seconds  of  time. 

Caroline,  These  three  minutes  fifty-six  seconds  is  the 
time  which  the  earth  takes  to  perform  the  additional  three 
hundred  and  sixty-fifth  part  of  the  circle,  in  order  to  bring  the 
same  meridian  back  to  the  sun. 

Mrs,  B,  Precisely.  Hence  the  stars  gain  every  day 
three  minutes  fifty-six  seconds  on  the  sun,  which  makes  them 
rise  that  portion  of  time  earlier  every  day. 

When  time  is  calculated  by  the  stars  it  is  called  sidereal 
time,  when  by  the  sun  solar  or  apparent  time.* 

Caroline,  Then  a  sidereal  day  is  three  minutes  fifty-six 
seconds  shorter  than  a  solar  day  of  twenty-four  hours. 

Mrs,  B,  I  must  also  explain  to  you  what  is  meant  by  a 
sidereal  year. 

The  common  year,  called  the  solar  or  tropical  year,  con- 
taining 365  days,  five  hours,  forty-eight  minutes,  and  fifty-two 
seconds,  is  measured  from  the  time  the  sun  sets  out  from  one 
of  the  equinoxes,  or  solstices,  till  it  returns  to  the  same  again  ; 
but  this  year  is  completed  before  the  earth  has  finished  one 
entire  revolution  in  its  orbit. 

Emily,  I  thought  that  the  earth  performed  one  complete 
revolution  in  its  orbit  every  year ;  what  is  the  reason  of  this 
variation  ? 

Mrs,  B,  It  is  owing  to  the  spheroidal  figure  of  the  earth. 
The  elevation  about  the  equator  produces  much  the  same 

*  If  one  clock  should  be  so  well  regulated  as  to  shew  the  time  to  be  XII 
at  noon  this  day,  and  on  the  365th  day  afterward  ;  and  another  clock 
should  be  so  well  regulated  as  to  show  the  time  to  be  XII  every  day  or 
night  when  any  given  star  is  on  the  meridian  ;  the  latter  clock  would 
gain  three  minutes,  fifty -five  seconds,  and  fifty-four  sixtieth  parts  of  a 
second  upon  the  former  in  each  revolution  of  the  same  star  to  the  met'i- 
dian. 

10* 


114  ON  THE  EARTH. 

eftect  as  if  a  similar  mass  of  matter,  collected  in  the  form  of  a 
moon,  revolved  round  the  equator.  When  this  moon  acted 
on  the  earth  in  conjunction  with  or  in  opposition  to  the  sun, 
variations  in  the  earth's  motion  would  be  occasioned,  and 
these  variations  produce  what  is  called  the  precession  of  the 
equinoxes* 

Emily,  What  does  that  mean  ?  I  thought  the  equinoctial 
points,  or  nodes,  were  fixeil  points  in  the  heavens,  in  which 
die  equator  cuts  the  ecliptic. 

Mrs,  B.  These  points  are  not  quite  fixed,  but  have  an 
apparently  retrograde  motion,  that  is  to  say,  instead  of  being 
every  revolution  in  the  same  place,  they  move  backwards. 
Thus  if  the  vernal  equinox  is  at  A,  (fig.  1.  plate  XI.)  the 
autumnal  one  will  be  at  B  instead  of  C,  and  the  following 
vernal  equinox  at  D  instead  of  at  A,  as  would  be  the  case  if 
the  equinoxes  were  stationary  at  opposite  points  of  the  earth's 
orbit. 

Caroline.  So  that  when  the  earth  moves  from  one  equi- 
nox to  the  other,  though  it  takes  half  a  year  to  perform  th^ 
journey,  it  has  not  travelled  through  half  its  orbit. 

Mrs,  B,  And,  consequently,  when  it  returns  again  to  the 
first  equinox,  it  has  not  completed  the  whole  of  its  orbit.  In 
order  to  ascertain  when  the  earth  has  performed  an  entire 
revolution  in  its  orbit,  we  must  observe  when  the  sun  returns 
in  conjunction  with  any  fixed  star  ;  and  this  is  called  a  side- 
real year.  Supposing  a  fixed  star  situated  at  E,  (fig.  1.  plate 
XI.)  the  sun  would  not  appear  in  conjunction  with  it  till  the 
earth  had  returned  to  A,  when  it  would  have  completed  its 
orbit. 

Emily,  And  how  much  longer  is  the  sidereal  than  the 
solar  year  ? 

Mrs,  B,  Only  twenty  minutes  ;  so  that  the  variation  of 
the  equinoctial  points  is  very  inconsiderable.  I  have  given 
them  a  greater  extent  in  the  figure  in  order  to  render  them 
sensible. 

In  regard  to  time,  I  must  further  add,  that  the  earth's  diu¥- 
pal  motion  on  an  inclined  axis,  together  with  its  annual  rev- 
olution in  an  elliptic  orbit,  occasions  so  much  comphcation  in 
its  motion,  as  to  produce  many  irregularities  ;  therefore,  true 
equal  time  cannot  be  measured  by  the  sun.  A  clock,  which 
was  always  perfectly  correct,  would  in  some  parts  of  the  year 
be  before  the  sun,  and  in  other  parts  after  it.  There  are  but 
ibur   periods  in  which  the   sun  and  a  perfect  clock  would' 


I.  >;f 


ON  THE  EARTH.  115 

agree,  which  is  the  15th  of  April,  the  l6th  of  June,  the  23d 
of  August,  and  the  24th  of  December. 

Emily,  And  is  there  any  considerable  difference  between 
solar  time  and  true  time  ? 

Mrs,  B,  The  greatest  difference  amounts  to  between 
fifteen  and  sixteen  minutes.  Tables  of  equation  are  con- 
structed for  the  purpose  of  pointing  out  and  correcting  these 
differences  between  solar  time  and  equal  or  mean  time,  which 
is  the  denomination  giTen  by  astronomers  to  true  time. 


CONVERSATION  IX. 


ON  THE  MOON. 

Of  the  Moon^s  Motion  ;  Phases  of  the  Moon  ;  Eclipses  of 
the  Moon  ;  Eclipses  of  Jupiter's  Moons  ;  Of  the  Lati- 
tude and  Longitude ;  Of  the  Transits  of  the  Inferior 
Planets  ;  Of  the  Tides, 


MRS.  B. 

We  shall  to-day  confine  our  attention  to  the  moon^  which 
offers  many  interesting  phenomena. 

The  moon  revolves  round  the  earth  in  the  space  of  about 
twenty-nine  days  and  a  half,  in  an  orbit  nearly  parallel  to 
that  of  the  earth,  and  accompanies  us  in  our  revolution  round 
the  sun. 

Emily.  Her  motion  then  must  be  rather  of  a  complicated 
nature  ;  for  as  the  earth  is  not  stationary,  but  advances  in 
her  orbit  whilst  the  moon  goes  round  her,  the  moon  must  pro- 
ceed in  a  sort  of  progressive  circle. 

Mrs,  B,  That  is  true ;  and  there  are  also  other  circum- 
stances which  interfere  with  the  simplicity  and  regularity  of 
the  moon's  motion,  but  which  are  too  intricate  for  you  to 
understand  at  present. 

The  moon  always  presents  the  same  face  to  us,  by  which 
it  is  evident  that  she  turns  but  once  upon  her  axis,  while  she 
performs  a  revolution  round  the  earth ;  so  that  the  inhabitants 
of  the  moon  have  but  one  day  and  one  night  in  the  course  of  a 
lunar  month. 

Caroline.  We  afford  them  however  the  advantage  of  a 
magnificent  moon  to  enlighten  their  long  nights. 

Mrs.  B*     That  advantage  is  but  partial ;  for  since  we  al- 


ON  THE  MOON.  Il7 

ways  see  the  same  hemisphere  of  the  moorij  the  inhabitants 
of  that  hemisphere  alone  can  perceive  us. 

Caroline,  One  half  of  the  moon  then  enjoys  our  light 
every  night,  while  the  other  half  has  constantly  nights  of 
darkness.  If  there  are  any  astronomers  in  those  regions, 
they  would  doubtless  be  tempted  to  visit  the  other  hemisphere, 
in  order  to  behold  so  grand  a  luminary  as  we  must  appear  to 
them.  But,  pray,  do  they  see  the  earth  under  all  the  changes 
which  the  moon  exhibits  to  us  ? 

M7's,  B,  Exactly  so.  These  changes  are  called  the  phases 
of  the  moon,  and  require  some  explanation.  In  figure  2,  plate 
XI.  let  us  say  that  S  represents  the  sun,  E  the  earth,  and  A 
BCD  the  moon  in  different  parts  of  her  orbit.  When  the 
moon  is  at  A,  her  dark  side  being  turned  towards  the  earth, 
we  shall  not  see  her  as  at  a  ;  but  her  disappearance  is  of  very 
short  duration,  and  as  she  advances  in  her  orbit  we  perceive 
her  under  the  form  of  a  new  moon  ;  when  she  has  gone 
through  one-eighth  of  her  orbit  at  B,  one  quarter  of  her  en- 
lightened hemisphere  will  be  turned  tow^ards  the  earth,  and 
she  will  then  appear  horned  as  at  b  ;  when  she  has  performed 
one  quarter  of  her  orbit,  she  shows  us  one  half  of  her  enlight- 
ened side  as  at  c  ;  at  c?  she  is  said  to  be  gibbous,  and  at  e  the 
whole  of  the  enlightened  side  appears  to  us,  and  the  moon  is  at 
full.  As  she  proceeds  in  her  orbit  she  becomes  again  gib- 
bous, and  her  enlightened  hemisphere  turns  gradually  away 
from  us  until  she  completes  her  orbit  and  disappears,  and 
then  again  resumes  her  form  of  a  new  moon. 

When  the  moon  is  at  full,  or  a  new  moon,  she  is  said  to  be 
in  conjunction  with  the  sun,  as  they  are  then  both  in  the  same 
direction  w^ith  regard  to  the  earth  ;  when  at  her  quarters  she 
is  said  to  be  in  opposition  to  the  sun. 

Emily*  Are  not  the  eclipses  produced  by  the  moon  pass- 
ing between  the  sun  and  the  earth  ? 

Mrs.  B.  Yes  ;  when  the  moon  passes  between  the  sun 
and  the  earth,  she  intercepts  his  rays,  or  in  other  words,  casts 
a  shadow  on  the  earth,  then  the  sun  is  eclipsed,  and  the  day- 
light gives  place  to  darkness,  while  the  moon's  shadow  is 
passing  over  us. 

When,  on  the  contrary,  the  earth  is  between  the  sun  and 
the  moon,  it  is  we  who  intercept  the  sun's  rays,  and  cast  a 
shadow  on  the  moon  ;  the  moon  is  then  darkened,  she  di-s 
appears  from  our  view,  and  is  eclipsed. 

Emily.     But  as  the  moon  goes  roynd  the   earth  every 


118  ON  THE  MOON. 

month,  she  must  be  once  during  that  time  between  the  earth 
and  the  sun,  and  the  earth  must  likewise  be  once  between  the 
sun  and  the  moon,  and  yet  we  have  not  a  solar  and  a  lunar 
eclipse  every  month  ? 

Mrs.  B,  The  orbits  of  the  earth  and  moon  are  not  exactly 
parallel,  but  cross  or  intersect  each  other  ;  and  the  moon 
generally  passes  either  above  or  below  the  earth  when  she  is 
in  conjunction  with  the  sun,  and  does  not  therefore  intercept 
the  sun's  rays,  and  produce  an  eclipse  ;  for  this  can  take 
place  only  when  the  earth  and  moon  are  in  conjunction  in 
that  part  of  their  orbits  which  cross  each  other,  (called  the 
nodes  of  their  orbits)  because  it  is  then  only,  that  they  are 
both  in  a  right  line  with  the  sun. 

Emily,  And  a  partial  eclipse  takes  place,  I  suppose,  when 
the  moon  in  passing  by  the  earth,  is  not  sufficiently  above  or 
below  the  earth's  shadow  entirely  to  escape  it  ? 

Mrs.  B.  Yes,  one  edge  of  her  disk  then  dips  into  the 
shadow,  and  is  eclipsed  ;  but  as  the  earth  is  larger  than  the 
moon,  when  the  eclipse  happens  precisely  at  the  nodes,  they 
are  not  only  total,  but  last  for  some  length  of  time. 

When  the  sun  is  eclipsed,  the  total  darkness  is  confined  to 
one  particular  part  of  the  earth,  evidently  showing  that  the 
moon  is  smaller  than  the  earth,  since  she  cannot  entirely 
skreen  it  from  the  sun.  In  fig.  1.  plate  XII.  you  will  find  a 
solar  eclipse  described ;  S  is  the  sun,  M  the  moon,  and  E  the 
earth ;  and  the  moon's  shadow,  you  see,  is  not  large  enough 
to  cover  the  earth.  The  lunar  eclipses  on  the  contrary  are 
visible  from  every  part  of  the  earth,  where  the  moon  is  above 
the  horizon  ;  and  we  discover  by  the  length  of  time  which 
the  moon  is  in  passing  through  the  earth's  shadow,  that  it 
would  be  sufficient  to  eclipse  her  totally,  were  she  47  times 
her  actual  size ;  it  follows,  therefore,  that  the  earth  is  47  times 
the  size  of  the  moon. 

In  fig.  2.  S  represents  the  sun,  which  pours  forth  rays  of 
light  in  straight  lines  in  every  direction.  E  is  the  earth,  and 
M  the  moon.  Now  a  ray  of  light  coming  from  one  extremity 
of  the  sun's  disk  in  the  direction  A  B,  will  meet  another  com- 
ing from  the  opposite  extremity  in  the  direction  C  B  ;  the 
shadow  of  the  earth  cannot  therefore  extend  beyond  B  ;  as 
the  sun  is  larger  than  the  earth,  the  shadow  of  the  latter  is 
conical,  or  the  figure  of  a  sugar  loaf ;  it  gradually  diminishes, 
and  is  much  smaller  than  the  earth  where  the  moon  passes 
through  it,  and  yet  we  find  the  moon  to  be  Bot  only  totally 


ON  THE  MOON.  119 

eclipsedj  but  some  length  of  time  in  darkness,  and  hence  we 
are  enabled  to  ascertain  its  real  dimensions. 

Emily,  When  the  moon  eclipses  the  sun  to  us,  we  must 
be  eclipsed  to  the  moon  ? 

Mrs,  B.  Certainly  ;  for  if  the  moon  intercepts  the  sun^s 
raysj  and  cast  a  shadow  on  us,  we  must  necessarily  disappear 
to  the  moon,  but  only  partially,  as  in  fig.  1. 

Caroline.  There  must  be  a  great  number  of  eclipses  in 
the  distant  planets,  which  have  so  many  moons  ? 

Mrs.  jB.  Yes,  few  days  pass  without  an  eclipse  taking 
place  ;  for  among  the  number  of  satellites,  one  or  other  of 
them  are  continually  passing  either  between  their  planet  and 
the  sun,  or  between  the  planet  and  each  other.  Astrono- 
mers are  so  well  acquainted  with  the  motion  of  the  planets 
and  their  satellites,  that  they  have  calculated  not  only  the 
eclipses  of  our  moon,  but  those  of  Jupiter,  with  such  perfect 
accuracy,  that  it  has  afforded  a  means  of  ascertaining  the 
longitude. 

Caroline,  But  is  it  not  very  easy  to  find  both  the  latitude 
and  longitude  of  any  place  by  a  map  or  globe  ? 

Mrs,  B,  If  you  know  where  you  are  situated,  there  is  no 
difiiculty  in  ascertaining  the  latitude  or  longitude  of  the  place 
by  referring  to  a  map  ;  but  supposing  that  you  had  been  a 
length  of  time  at  sea,  interrupted  in  your  course  by  storms,  a 
map  would  afford  you  very  little  assistance  in  discovering 
where  you  were.  - 

Caroline,  Under  such  circumstances,  I  confess  I  should 
be  equally  at  a  loss  to  discover  either  latitude  or  longitude. 

Mrs.  B,  The  latitude  may  be  easily  found  by  taking  the 
-altitude  of  the  pole ;  that  it  is  to  say  the  number  of  degrees 
that  it  is  elevated  above  the  horizon,  for  the  pole  appears 
more  elevated  as  we  approach  it,  and  less  as  we  recede 
from  it. 

Caroline,  But  unless  you  can  see  the  pole  how  can  you 
take  its  altitude  ? 

Mrs.  B.  The  north  pole  points  constantly  towards  one 
particular  part  of  the  heavens  in  which  a  star  is  situated,  call- 
ed the  Polar  Star :  this  star  is  visible  on  clear  nights,  from 
every  part  of  the  northern  hemisphere,  the  altitude  of  the 
polar  star,  is  therefore  the  same  number  of  degrees  as  that  of 
the  pole  ;  the  latitude  may  also  be  determined  by  observa- 
tions made  on  the  sun  or  any  of  the  fixed  stars  ;  the  situation 
therefore  of  a  vessel  at  sea,  with  regard  to  north  and  south, 


120  as  THE  MOON. 

is  easily  ascertained.  The  difficulty  is  respecting  east  and 
west,  that  is  to  say  its  longitude.  As  we  have  no  eastern 
poles  from  which  we  can  reckon  our  distance  ;  some  particu- 
lar spot  must  be  fixed  upon  for  that  purpose.  The  English 
reckon  from  the  meridian  of  Greenwich,  where  the  royal 
observatory  is  situated ;  in  French  maps  you  will  find  that 
the  longitude  is  reckoned  from  Paris. 

The  rotation  of  the  earth  on  its  axis  in  24  hours  from 
west  to  east  occasions,  you  know,  an  apparent  motion  of  the 
sun  and  stars  in  the  contrary  direction,  and  the  sun  appears 
to  go  round  the  earth  in  the  space  of  24  hours,  passing  over 
fifteen  degrees  or  a  twenty-fourth  part  of  the  earth's  circum- 
ference every  hour ;  therefore,  when  it  is  twelve  o'clock  in 
London,  it  is  one  o'clock  in  any  place  situated  fifteen  degrees 
to  the  east  of  London,  as  the  sun  must  have  passed  the  meri- 
dian of  that  place  an  hour  before  he  reaches  that  of  London. 
For  the  same  reason  it  is  eleven  o'clock  to  any  place  situated 
fifteen  degrees  to  the  west  of  London,  as  the  sun  will  not 
come  to  that  meridian  till  an  hour  later. 

If  then  the  captain  of  a  vessel  at  sea,  could  know  precisely 
what  was  thtB  hour  at  London,  he  could,  by  looking  at  his 
watch,  and  comparing  it  w  ith  the  hour  of  the  spot  in  which  he 
was,  ascertain  the  longitude. 

Emily,  But  if  he  had  not  altered  his  watch,  since  he  sail- 
ed from  London,  it  would  indicate  the  hour  it  was  then  in 
London. 

Mrs.  B.  True  ;  but  in  order  to  know  the  hour  of  the  day 
of  the  spot  in  which  he  is,  the  captain  of  a  vessel  regulates  his 
watch  by  th€  sun  when  it  reaches  the  meridian. 

Emily,  Then  if  he  had  two  watches,  he  might  keep  one 
regulated  daily,  and  leave  the  other  unaltered  ;  the  former 
would  indicate  the  hour  of  the  place  in  which  he  was  situated, 
and  the  latter  the  hour  of  London  ;  and  by  comparing  them 
together,  he  would  be  able  to  calculate  his  longitude. 

Mrs,  B.  You  have  discovered,  Emily,  a  mode  of  finding 
the  longitude,  which  I  have  the  pleasure  to  tell  you,  is  univer- 
sally adopted  :  watches  of  a  superior  construction,  called 
chronometers,  or  time-keepers,  are  used  for  this  purpose ;  but 
the  best  watches  are  liable  to  imperfections,  and  should  the 
time-keeper  go  too  fast,  or  too  slow,  there  would  be  no  means 
of  ascertaining  the  error ;  implicit  reliance  cannot  consequently 
be  placed  upon  them. 

Recourse  is  therefore  had  to  the  eclipses  of  Jupiter's  satel- 


ON  THE  MOON.  121 

lites.  A  table  is  made  of  the  precise  tinie  at  which  the  several 
moons  are  eclipsed  to  a  spectator  at  London ;  when  they 
appear  eclipsed  to  a  spectator  in  any  other  spot,  he  may,  by 
consulting  the  table,  know  what  is  the  hour  at  London  ;  for 
the  eclipse  is  visible  at  the  same  moment  from  whatever  place 
on  the  earth  it  is  seen.  He  has  then  only  to  look  at  the  watch 
which  points  out  the  hour  of  the  place  in  which  he  is,  and  by 
observing  the  difference  of  time  there,  and  at  London,  he  may 
immediately  determine  his  longitude. 

Let  us  suppose,  that  a  certain  moon  of  Jupiter  is  always 
eclipsed  at  six  o'clock  in  the  evening ;  and  that  a  man  at  sea 
consults  his  watch,  and  finds  that  it  is  ten  o'clock,  at  night, 
where  he  is  situated,  at  the  moment  the  eclipse  takes  place  ; 
what  will  be  his  longitude  ? 

Emily,  That  is  four  hours  later  than  in  London  :  four 
times  fifteen  degrees  makes  60  ;  he  would,  therefore,  be  sixty 
degrees  east  of  London,  for  the  sun  must  have  passed  his 
meridian  before  it  reaches  that  of  London. 

Mrs.  B.  For  this  reason  the  hour  is  always  later  than  in 
London,  when  the  place  is  east  longitude,  and  earlier  when  it 
is  west  longitude.  Thus  the  longitude  can  be  ascertained 
whenever  the  eclipses  of  Jupiter's  moons  are  visible. 

But  it  is  not  only  the  secondary  planets  which  produce 
eclipses,  for  the  primary  planets  near  the  sun  eclipse  him  to 
those  at  a  greater  distance  when  they  come  in  conjunction  in 
the  nodes  of  their  orbits  ;  but  as  the  primary  planets  are 
much  longer  in  performing  their  course  round  the  sun,  than 
the  satellites  in  going  round  their  primary  planets,  these 
eclipses  very  seldom  occur.  Mercury  and  Venus  have  how- 
ever passed  in  a  right  line  between  us  and  the  sun,  but  being 
at  so  great  a  distance  from  us,  their  shadows  did  not  extend  so 
far  as  the  earth  ;  no  darkness  was  therefore  produced  on  any 
part  of  our  globe  ;  but  the  planet  appeared  like  a  small  black 
spot,  passing  across  the  sun's  disk  ;  this  is  called  a  transit  of 
the  planet. 

It  was  by  the  last  transit  of  Venus,  that  astronomers  were 
enabled  to  calculate  with  some  degree  of  accuracy  the  dis- 
tance of  the  earth  from  the  sun,  and  the  dimensions  of  the 
latter. 

Emily.  I  have  heard  that  the  tides  are  affected  by  the 
moon,  but  I  cannot  conceive  what  influence  it  can  have  on 
them. 

11 


122  ON  THE  MOON. 

Mrs.  B,  They  are  produced  by  the  moon's  attraction , 
which  draws  up  the  waters  in  a  protuberance. 

Caroline,  Does  attraction  act  on  water  more  powerfully 
than  on  land  ?  I  should  have  thought  it  would  have  been  just 
the  contrary,  for  land  is  certainly  a  more  dense  body  than 
water  ? 

Mrs,  B,  Tides  do  not  arise  from  water  being  more  strong- 
ly attracted  than  land,  for  this  certainly  is  not  the  case  ;  but 
the  cohesion  of  jfluids  being  much  less  than  that  of  solid  bodies, 
they  more  easily  yield  to  the  power  of  gravity,  in  consequence 
of  which  the  waters  immediately  below  the  moon  are  drawn 
up  by  it  in  a  protuberance,  producing  a  full  tide,  or  what  is 
commonly  called  high  water,  at  the  spot  where  it  happens. 
So  far  the  theory  of  the  tides  is  not  difficult  to  understand. 

Caroline.  On  the  contrary,  nothing  can  be  more  simple ; 
the  waters,  in  order  to  rise  up  under  the  moon,  must  draw 
the  waters  from  the  opposite  side  of  the  globe,  and  occasion 
ebb-tide,  or  low  water  in  those  parts. 

Mrs.  B.  You  draw  your  conclusion  rather  too  hastily,  my 
dear ;  for  according  to  your  theory,  we  should  have  full  tide 
only  once  in  twenty-four  hours,  that  is,  every  time  that  we 
were  below  the  moon,  while  we  find  that  we  have  two  tides  in 
the  course  of  twenty-four  hours,  and  that  it  is  high  water  with 
us  and  with  our  antipodes  at  the  same  time. 

Caroline.  Yet  it  must  be  impossible  for  the  moon  to  at- 
tract the  sea  in  opposite  parts  of  the  globe,  and  in  opposite 
directions  at  the  same  time. 

Mrs.  B.  This  opposite  tide  is  rather  more  difficult  to 
explain,  than  that  which  is  drawn  up  beneath  the  moon  ; 
with  a  little  attention,  however,  I  hope  I  shall  be  able  to  make 
you  understand  it. 

You  recollect  that  the  earth  and  moon  are  mutually  attrac- 
ted towards  a  point,  their  common  centre  of  gravity  and  of 
motion  ;  can  you  tell  me  what  it  is  that  prevents  their  meeting 
and  uniting  at  this  point  ? 

Emily.  Their  projectile  force,  which  gives  them  a  tenden- 
cy to  fly  from  this  centre. 

Mrs.  B.  And  is  hence  called  their  centrifugal  force. 
Now  we  know  that  the  centrifugal  force  increases  in  proportion 
to  the  distance  from  the  centre  of  motion. 

Caroline.  Yes,  I  recollect  your  explaining  that  to  us,  and 
illustrating  it  by  the  motion  of  the  flyers  of  a  wmd-mill,  and 
the  spinning  of  a  top. 


ON  THE  MOUN.  123 

Emily,  And  it  was  but  the  other  day  you  showed  us  that 
bodies  weighed  less  at  the  equator,  than  in  the  polar  regions, 
in  consequence  of  the  increased  centrifugal  force  in  the  equa- 
torial parts. 

Mrs,  B.  Very  well.  The  power  of  attraction,  on  the 
contrary,  increases  as  the  distance  from  the  centre  of  gravity 
diminishes  ;  when,  therefore,  the  two  centres  of  gravity  and 
of  motion  are  in  the  same  spot,  as  is  the  case  with  regard  to 
the  moon  and  the  earth,  the  centrifugal  power  and  those  of 
attraction,  will  be  in  inverse  proportion  to  each  other  ;  that  is 
to  say,  where  the  one  is  strongest,  the  other  will  be  weakest. 

Emily,  Those  parts  of  the  ocean,  then,  which  are  most 
strongly  attracted  will  have  least  centrifugal  force  ;  and  those 
parts  which  are  least  attracted,  will  have  the  greatest  centri- 
fugal force. 

Mrs,  B,  In  order  to  render  the  question  more  simple,  let 
us  suppose  the  earth  to  be  every  where  covered  by  the  ocean, 
as  represented  in  (fig.  3.  pi.  XII.)  M  is  the  moon,  A  B  C  D 
the  earth,  and  X  the  common  centre  of  gravity  and  of  motion 
of  these  two  planets.  Now  the  waters  on  the  surface  of  the 
earth,  about  A,  being  more  strongly  attracted  than  any  other 
part,  will  be  elevated  ;  the  attraction  of  the  moon  at  B  and  C 
being  less,  and  at  D  least  of  all.  But  the  centrifugal  force  at 
D,  will  be  greatest,  and  the  waters  there,  will  in  consequence 
have  the  greatest  tendency  to  recede  from  the  moon  ;  the 
waters  at  B  and  C  will  have  less  tendency  to  recede,  and  at 
A  least  of  all.  The  waters,  therefore,  at  D,  will  recede 
furthest,  at  the  same  time  that  they  are  least  attracted,  and  in 
consequence  will  be  elevated  in  a  protuberance  similar  to  that 
at  A. 

Emily,  The  tide  A,  then,  is  produced  by  the  moon's  at- 
traction, and  increased  by  the  feebleness  of  the  centrifugal 
power  in  those  parts  ;  and  the  tide  D  is  produced  by  the 
centrifugal  force,  and  increased  by  the  feebleness  of  the  moon's 
attraction  in  those  parts. 

Caroline,  And  when  it  is  high  water  at  A  and  D,  it  is 
low  water  at  B  and  C  :  now  I  think  I  comprehend  the  nature 
of  the  tides  again,  though  I  confess  it  is  not  quite  so  easy  as 
I  at  first  thought. 

But,  Mrs.  B.,  why  does  not  the  sun  produce  tides  as  well 
as  the  moon  ;  for  its  attraction  is  greater  than  that  of  the 
moon  ? 

Nrs.  B,     It  would  be  at  an  equal  distance,  but  our  vicini- 


124  ON  THE  ^lOOX. 

ty  to  the  moon  makes  her  influence  more  powerful.  The  sun 
has,  however,  a  considerable  effect  on  the  tides,  and  increases 
or  diminishes  them  as  it  acts  in  conjunction  with,  or  in  oppo- 
sition to  the  moon. 

Etnily.     I  do  not  quite  understand  that. 

Mrs.  B.  The  moon  is  a  month  in  going  round  the  earth  ; 
twice  during  that  time,  therefore,  at  full  and  at  change,  she  is 
in  the  same  direction  as  the  sun,  both  then  act  in  conjufiction 
on  the  earth,  and  produce  very  great  tides,  called  spring  tides, 
as  described  in  fig.  4.  at  A  and  B  ;  but  when  the  moon  is  at 
the  intermediate  parts  of  her  orbit,  the  sun,  instead  of  afford- 
ing assistance,  weakens  her  power  by  acting  in  opposition  to 
it ;  and  smaller  tides  are  produced,  called  neap  tides,  as  rep- 
resented in  fig.  5. 

Emihj.  I  have  often  observed  the  difference  of  these  tides 
when  I  have  been  at  the  sea  side. 

But  since  attraction  is  mutual  between  the  moon  and  the 
earth,  we  must  produce  tides  in  the  moon  ;  and  these  must 
be  more  considerable  in  proportion  as  our  planet  is  larger. 
And  3^et  the  moon  does  not  appear  of  an  oval  form. 

Mrs.  B.  You  must  recollect,  that  in  order  to  render  the 
explanation  of  the  tides  clearer,  we  suppose  the  whole  sur- 
face of  the  earth  to  be  covered  with  the  ocean  ;  but  that  is 
not  really  the  case,  either  with  the  earth  or  the  moon,  and 
the  land  which  intersects  the  water  destroys  the  regularity  of 
the  effect. 

Caroline,  True  ;  we  may,  however,  be  certain,  that 
wiienever  it  is  high  water  the  moon  is  immediately  over  our 
heads. 

Mrs.  B.  Not  so  either  ;  for  as  a  similar  effect  is  produc- 
ed on  that  part  of  the  globe  immediately  beneath  the  moon, 
and  on  that  part  most  distant  from  it,  it  cannot  be  over  the 
heads  of  the  inhabitants  of  both  those  situations  at  the  same 
time.  Besides,  as  the  orbit  of  the  moon  is  very  nearly  paral- 
lel to  that  of  the  earth,  she  is  never  vertical  but  to  the  inhab- 
itants of  the  torrid  zone  ;  in  that  climate,  therefore,  the  tides 
are  greatest  and  they  diminish  as  you  recede  from  it  and 
approach  the  poles. 

Caroline.  In  the  torrid  zone,  then,  I  hope  you  will  grant 
that  the  moon  is  immediately  over,  or  opposite  the  spots 
where  it  is  high  water  ? 

Mrs.  B.  I  cannot  even  admit  that ;  for  the  ocean  natur- 
ally partaking  of  the  earth's  motion,  in  its  rotation  from  west 


ON  THE  MOO.V.  125 

fo  east,  the  moon,  in  forming  a  tide,  has  to  contend  against 
the  eastern  motion  of  the  waves.  All  matter,  you  know, 
by  its  inertia,  makes  some  resistance  to  a  change  of  state  : 
the  waters,  therefore,  do  not  readily  yield  to  the  attraction  of 
the  moon,  and  the  effect  of  her  influence  is  not  complete  till 
three  hours  after  she  has  passed  the  meridian,  where  it  is 
full  tide. 

Emily,  Pray  what  is  the  reason  that  the  tide  is  three- 
quarters  of  an  hour  later  every  day  ? 

Mrs.  B.  Because  it  is  twenty-four  hours  and  three-quar- 
ters before  the  same  meridian  on  our  globe  returns  beneath 
the  moon.  The  earth  revolves  on  its  axis  in  about  twenty- 
four  hours  ;  if  the  moon  were  stationary,  therefore,  the  same 
part  of  our  globe  would,  every  twenty-four  hours,  return 
beneath  the  moon  ;  but  as  during  our  daily  revolution  the 
moon  advances  in  her  orbit,  the  earth  must  make  more  than 
a  complete  rotation  in  order  to  bring  the  same  meridian 
opposite  the  moon  :  we  are  three-quarters  of  an  hour  in 
overtaking  her.  The  tides,  therefore,  are  retarded  for  the 
same  reason  that  the  moon  rises  later  by  three-quai'ters  of  an 
hour  every  day. 

We  have  now,  I  think,  concluded  the  observations  I  had 
to  make  to  you  on  the  subject  of  astronomy  ;  at  our  next 
interview,  I  shall  attempt  to  explain  to  you  the  elements  of 
hydrostatics. 


11^ 


CONVERSATION  X. 


ON  THE  MECHANICAL  PROPERTIES  OF 
FLUIDS. 

Definition  of  a  Fluid ;  Distinction  between  Fluids  and 
Liquids ;  Of  Non-Elastic  Fluids  ;  Scarcely  Susceptible 
of  Cojupression  ;  Of  the  Cohesion  of  Fluids  ;  Of  their 
Gravitation  ;  Of  their  Equilibrium  ;  Of  their  Pressure  ; 
Of  Specific  Gravity  ;  Of  the  Specific  Gravity  of  Bodies 
Heavier  than  Water  ;  Of  those  of  the  Same  Weight  as 
Water  ;  Of  those  Lighter  than  Water  ;  Of  the  Specific 
Gravity  of  Fluids. 


MRS.  B. 

We  have  hitherto  eonfined  our  attention  to  the  mechanical 
properties  of  sohd  bodies,  which  have  been  illustrated,  and,  I 
hope,  thoroughly  impressed  upon  your  memory,  by  the  con- 
versations we  have  subsequently  had  on  astronomy.  It  will 
now  be  necessary  for  me  to  give  you  some  account  of  the 
mechanical  properties  of  fluids — a  science  which  is  called 
hydrostatics.  A  fluid  is  a  substance  which  yields  to  the 
slightest  pressure.  If  you  dip  your  hand  into  a  basin  of  water, 
you  are  scarcely  sensible  of  meeting  with  any  resistance. 

Emily,  The  attraction  of  cohesion  is,  then,  I  suppose, 
less  powerful  in  fluids  than  in  solids  ? 

Mrs,  B.  Yes  ;  fluids,  generally  speaking,  are  bodies  of 
less  density  than  solids.  From  the  slight  cohesion  of  the 
particles  of  fluids,  and  the  facility  with  which  they  slide  over 
each  other,  it  is  inferred,  that  they  must  be  small,  smooth,  and 
globular  ;  smooth,  because  there  appears  to  be  little  or  no 
friction  among  them  :  and  globular,  because  touching  each 


MECHANICAL  PROPERTIES  OF  FLUID*.  12? 

Other  but  by  a  point  would  account  for  the  slightness  of  their 
cohesion.* 

Caroline.  Pray  what  is  the  distinction  between  a  fluid 
and  a  Hquid  ? 

Mrs,  B*  Liquids  comprehend  only  one  class  of  fluids. 
There  is  another  class  distinguished  by  the  name  of  elastic 
fluids,  or  gases,  which  comprehends  the  air  of  the  atmosphere, 
and  all  the  various  kinds  of  air  with  which  you  will  become 
acquainted  when  you  study  chemistry.  Their  mechanical 
properties  we  shall  examine  at  our  next  meeting,  and  confine 
our  attention  this  morning  to  those  of  liquids,  or  non-elastic 
fluids. 

Water,  and  liquids  in  general,  are  scarcely  susceptible  of 
being  compressed,  or  squeezed  into  a  smaller  space  than  that 
which  they  naturally  occupy.  This  is  supposed  to  be  owing 
to  the  extreme  minuteness  of  their  particles,  which,  rather 
than  submit  to  compression,  force  their  way  through  the  pores 
of  the  substance  which  confines  them.  This  was  show^n  by  a 
celebrated  experiment,  made  at  Florence  many  years  ago. 
A  hollow  globe  of  gold  was  filled  with  water,  and  on  its 
being  submitted  to  great  pressure,  the  water  was  seen  to 
exude  through  the  pores  of  the  gold,  which  it  covered  with  a 
fine  dew.  Fluids  gravitate  in  a  more  perfect  manner  than 
solid  bodies  ;  for  the  strong  cohesive  attraction  of  the  parti- 
cles of  the  latter  in  some  measure  counteracts  the  efiects  of 
gravity.  In  this  table,  for  instance,  the  cohesion  of  the 
particles  of  wood  enables  four  slender  legs  to  support  a 
considerable  weight.  Were  the  cohesion  destroyed,  or,  in 
other  words,  the  wood  converted  into  a  fluid,  no  support 
could  be  afforded  by  the  legs,  for  the  particles  no  longer 
cohering  together,  each  would  press  separately  and  independ- 
ently, and  would  be  brought  to  a  level  with  the  surface  of  the 
earth. 

Emily,  This  want  of  cohesion  is  then  the  reason  why 
fluids  can  never  be  formed  into  figures,  or  maintained  in 

*  If  the  particles  of  fluids  are  round,  there  must  be  vacant  spaces  be- 
tween them,  in  the  same  manner  as  there  are  vacuities  between  cannon 
balls  that  are  piled  toj^ether ;  between  these  balls  smaller  shot  may  be 
placed,  and  between  these,  others  still  smaller,  or  gravel,  or  sand,  may  be 
diffused.  In  a  similar  manner,  a  certain  quantity  of  parfcles  of  sugar 
can  be  taken  up  in  water  without  increasing  its  bulk,  and  when  the  water 
has  dissolved  the  sugar,  salt  may  be  dissolved  in  it,  and  yet  the  bulk  re- 
main the  same  ;  arid  admitting  that  the  particles  of  water  are  round,  this 
33  easily  accounted  for. 


128  MECHANICAL  PROPERTIES  OF  FLUIDS. 

heaps  ;  for  thougli  it  is  true  the  wind  raises  water  into  waves^^ 
they  are  immediately  afterwards  destroyed  by  gravity,  and 
water  always  finds  its  level. 

M7S,  B,  Do  you  understand  what  is  meant  by  the  level, 
or  equilibrium  of  fluids  ? 

Emily.  I  believe  I  do,  though  I  feel  rather  at  a  loss  to 
explain  it.  Is  not  a  fluid  level  when  its  surface  is  smooth  and 
flat,  as  is  the  case  with  all  fluids  when  in  a  state  of  rest  ? 

Mrs.  B.  Smooth,  if  you  please,  but  not  flat  ;  for  the 
definition  of  the  equilibrium  of  a  fluid  is,  that  every  part  of 
the  surface  is  equally  distant  from  the  point  to  which  gravity 
tends,  that  is  to  say,  from  the  centre  of  the  earth  ;  hence  the 
surface  of  all  fluids  must  be  bulging,  not  flat,  since  they  will 
partake  of  the  spherical  form  of  the  globe.  This  is  very  evi- 
dent in  large  bodies  of  water,  such  as  the  ocean,  but  the 
sphericity  of  small  bodies  of  water  is  so  trifling,  that  their 
surfaces  appear  flat. 

This  level,  or  equilibrium  of  fluids,  is  the  natural  result  of 
dieir  particles  gravitating  independently  of  each  other ;  for 
when  any  particle  of  a  fl  jid  accidentally  finds  itself  elevated 
above  the  rest,  it  is  attracted  down  to  the  level  of  the  surface 
of  the  fluid,  and  the  readiness  with  which  fluids  yield  to  the 
slightest  impression,  will  enable  the  particle  by  its  weight  to 
penetrate  the  surface  of  the  fluid  and  mix  with  it. 

Caroline.  But  I  have  seen  a  drop  of  oil  float  on  the  sur- 
face of  water  without  mixing  with  it. 

Mrs.  B.  That  is,  because  oil  is  a  lighter  liquid  than  wa- 
ter. If  you  were  to  pour  water  over  it,  the  oil  would  rise  to 
the  surface,  being  forced  up  by  the  superior  gravity  of  the 
water.  Here  is  an  instrument  called  a  water-level,  (fig.  1. 
plate  XIII.)  which  is  constructed  upon  the  principle  of  the 
equilibrium  of  fluids.  It  consists  of  a  short  tube,  A  B, 
closed  at  both  ends,  and  containing  a  little  water  ;  when  the 
tube  is  not  perfectly  horizontal  the  water  runs  to  the  lower 
end,  and  it  is  by  this  means  that  the  level  of  any  situation  to 
which  we  apply  the  instrument,  is  ascertained. 

Solid  bodies  you  may,  therefore,  consider  as  gravitating  in 
masses,  for  the  strong  cohesion  of  their  particles  makes  them 
weigh  altogether,  while  every  particle  of  a  fluid  may  be  con- 
sidered as  composing  a  separate  mass,  gravitating  independ- 
ently of  each  other.  Hence  the  resistance  of  a  fluid  is  con- 
siderably less  than  that  of  a  solid  body  ;  for  the  resistance  of 
the  particles  acting  separately,  they  are  more  easily  overcome = 


.MECHANICAL  PROPERTIES  OF  FLUIDS.  129 

Emily.  A  body  of  water,  in  falling,  does  certainly  less 
injury  than  a  solid  body  of  the  same  weight. 

Mrs,  B.  The  particles  of  fluids  acting  thus  independently, 
press  against  each  other  in  every  direction,  not  only  down- 
wards but  upwards,  and  laterally  or  sideways  ;  and  in  conse- 
quence of  this  equality  of  pressure,  every  particle  remains  at 
rest  in  the  fluid.  If  you  agitate  the  fluid  you  disturb  this 
equahty  of  pressure  and  the  fluid  will  not  rest  till  its  equili- 
brium is  restored. 

Caroline,  The  pressure  downwards  is  very  natural  ;  it  is 
the  eflect  of  gravity,  one  particle  weighing  upon  another  pres- 
ses on  it ;  but  the  pressure  sideways,  and  particularly  the 
pressure  upwards,  I  cannot  understand. 

Mrs,  B,  If  there  were  no  lateral  pressure,  water  would 
not  run  out  of  an  opening  on  the  side  of  a  vessel.  If  you  fill 
a  vessel  with  sand,  it  will  not  run  out  of  such  an  opening, 
because  there  is  scarcely  any  lateral  pressure  among  its 
particles. 

Emily,  When  water  runs  out  of  the  side  of  a  vessel,  is  it 
not  owing  to  the  w^eight  of  the  water  above  the  opening  ? 

Mrs,  B,  If  the  particles  of  fluids  were  arranged  in  regular 
columns  thus,  (fig.  2.)  there  would  be  no  lateral  pressure,  for 
when  one  particle  is  perpendicularly  above  the  other,  it  can 
only  press  it  downwards  ;  but  as  it  must  continually  happen, 
that  a  particle  presses  between  two  particles  beneath,  (fig.  3.) 
these  last  must  sufler  a  lateral  pressure. 

Emily,  The  same  as  when  a  wedge  is  driven  into  a  piece 
of  wood,  and  separates  the  parts  laterally. 

Mrs.  B.  Yes.  Tlie  lateral  pressure  proceeds,  therefore, 
entirely  from  the  pressure  downwards,  or  the  weight  of  the 
liquid  above  ;  and  consequently  the  lower  the  orifice  is  made 
in  the  vessel,  the  greater  will  be  the  velocity  of  the  water 
rushing  out  of  it.  Here  is  a  vessel  of  water  (fig.  4.)  with 
three  stop  cocks  at  different  heights ;  we  shall  open  them, 
and  you  w^ill  see  with  what  different  degrees  of  velocity  the 
water  issues  from  them.     Do  you  understand  this,  Caroline?* 

Caroline,  Oh  yes.  The  water  from  the  upper  spout 
receiving  but  a  slight  pressure,  on  account  of  its  vicinity  to 

*  An  empty  bottle  Leing  corked,  and,  by  means  of  a  weight,  let  down 
a  certain  depth  into  the  sea,  it  wiil  be  broken,  or  the  cork  will  be  driven 
into  it  by  the  perpendicular  pressure.  But  a  bottle  filled  with  water  or 
any  other  liquid  may  be  let  down  to  any  depth,  without  damage,  because 
in  this  case  the  internal  pressure  is  equal  to  the  externa!. 


130  MECHANICAL  PROPERTIES  OP  FLUIDS. 

the  surface,  flows  but  gently  ;  the  second  cock  having  a 
greater  weight  above  it,  the  water  is  forced  out  with  greater 
velocity,  whilst  the  lowest  cock  being  near  the  bottom  of  the 
vessel,  receives  the  pressure  of  almost  the  whole  body  of  water, 
and  rushes  out  with  the  greatest  impetuosity. 

Mrs,  B»  Very  well  ;  and  you  must  observe,  that  as  the 
lateral  pressure  is  entirely  owing  to  the  pressure  downwards, 
it  is  not  effected  by  the  horizontal  dimensions  of  the  vessel, 
which  contains  the  water,  but  merely  by  its  depth ;  for  as 
every  particle  acts  independently  of  the  rest,  it  is  only  the 
column  of  particles,  immediately  above  the  orifice  that  can 
weigh  upon  and  press  out  the  water. 

Emily.  The  breadth  and  width  of  the  vessel  then  can  be 
of  no  consequence  in  this  respect.  The  lateral  pressure  on 
one  side,  in  a  cubical  vessel,  is,  I  suppose,  not  so  great  as  the 
pressure  downwards. 

Mrs.  B.  No,  in  a  cubical  vessel  the  pressure  downwards 
will  be  double  the  lateral  pressure  on  vne  side  ;  for  every 
particle  at  the  bottom  of  the  vessel  is  pressed  upon  by  a  col- 
umn of  the  whole  depth  of  the  fluid,  whilst  the  lateral  pressure 
diminishes  from  the  bottom  upwards  to  the  surface,  where  the 
particles  have  no  pressure. 

Caroline.  And  from  whence  proceeds  the  pressure  of 
fluids  upwards  ?  that  seems  to  me  the  most  unaccountable,  as 
it  is  in  direct  opposition  to  gravity. 

Mrs.  B.  And  yet  it  is  a  consequence  of  their  pressure 
downwards.  When,  for  example,  you  pour  water  into  a  tea- 
pot, the  w^ater  rises  in  the  spout  to  a  level  with  the  water  in 
the  pot.  The  particles  of  water  at  the  bottom  of  the  pot  are 
pressed  upon  by  the  particles  above  them  ;  to  this  pressure 
they  will  yield,  if  there  is  any  mode  of  making  way  for  the 
superior  particles,  and  as  they  cannot  descend,  they  will  change 
their  direction  and  rise  in  the  spout. 

Suppose  the  tea-pot  to  be  filled  with  columns  of  particles 
of  water  similar  to  that  described  in  fig.  4.  the  particle  1  at 
the  bottom  will  be  pressed  laterally  by  the  particle  2,  and  by 
this  pressure  be  forced  into  the  spout,  where,  meeting  w^ith 
the  particle  3,  it  presses  n  upwards  and  this  pressure  will  be 
continued,  from  3  to  4,  from  4  to  5,  and  so  on  till  the  water 
in  the  spout  has  risen  to  a  level  with  that  in  the  pot. 

Emilf/.  If  it  were  not  for  this  pressure  upwards,  forcing 
the  water  to  rise  in  the  spout,  the  equilibrium  of  the  fluid  would 
be  destroved. 


MECHANICAL  PROPERTIES   OF  FLUIDS.  131 

Caroline.  True ;  but  then  a  tea-pot  is  wide  and  large^ 
and  the  weight  of  so  great  a  body  of  water  as  the  pot  will  con- 
tain, may  easily  force  up  and  support  so  small  a  quantity  as 
will  fill  the  spout.  But  would  the  same  effect  be  produced  if 
the  spout  and  the  pot  were  of  equal  dimensions  ? 

Mrs,  B.  Undoubtedly  it  would.  You  may  even  reverse 
the  experiment  by  pouring  water  into  the  spout,  and  you  will 
find  that  the  water  will  rise  in  the  pot  to  a  level  with  that  in 
the  spout;  for  the  pressure  of  the  small  quantity  of  water  in  the 
spout  will  force  up  and  support  the  larger  quantity  in  the  pot. 
In  the  pressure  upwards,  as  well  as  that  laterally,  you  see 
that  the  force  of  pressure  depends  entirely  on  the  height,  and 
is  quite  independent  of  the  horizontal  dimensions  of  the  fluid. 

As  a  tea-pot  is  not  transparent,  let  us  try  the  experiment 
by  filling  this  large  glass  goblet  by  means  of  this  narrow  tube, 

(fig-  6.) 

Caroline,  Lobit,  Emily,  as  Mrs.  B.  fills  it,  how  the  water 
rises  in  the  goblet,  to  maintain  an  equilibrium  with  that  in  the 
tube. 

Now,  Mrs.  B.,  will  you  let  me  fill  the  tube  by  pouring  water 
into  the  goblet  ? 

Mrs.  B.  That  is  impossible.  However,  you  may  try  the 
experiment,  and  I  doubt  not  but  that  you  will  be  able  to  account 
for  its  failure. 

Caroline.  It  is  very  singular,  that  if  so  small  a  column  of 
water  as  is  contained  in  the  tube  can  force  up  and  support  the 
whole  contents  of  the  goblet  ;  that  the  weight  of  all  the  water 
in  the  goblet  should  not  be  able  to  force  up  the  small  quantity 
required  to  fill  the  tube  : — oh,  I  see  now  the  reason,  the  water 
in  the  goblet  cannot  force  that  in  the  tube  above  its  level,  and 
as  the  end  of  the  tube  is  considerably  higher  than  the  goblet, 
it  can  never  be  filled  by  pouring  water  into  the  goblet. 

Mrs.  B.  And  if  you  continue  to  pour  water  into  the  gob- 
let when  it  is  full,  the  water  will  run  over  instead  of  rising 
above  the  level  in  the  tube. 

I  shall  now  explain  to  you  the  meaning  of  the  specific  grav^ 
ity  of  bodies. 

Caroline.  What !  is  there  another  species  of  gravity  with 
which  we  are  not  yet  acquainted  ? 

Mrs.  B.  No  ;  the  specific  gravity  of  a  body,  means  sim- 
ply its  weight  compared  with  that  of  another  body  of  the  same 
size.     When  we  say,  that  substances  such  as  lead  and  stones 


132  MECHANICAL  PROPERTIES  OF  FLUIDS. 

are  heavy,  and  that  others,  such  as  paper  and  feathers,  are 
light,  we  speak  comparatively  ;  that  is  to  say,  that  the  first 
are  heavy,  and  the  latter  light,  in  comparison  with  the  general- 
ity of  substances  in  nature.  Would  you  call  w^ood  and  chalk 
light  or  heavy  bodies  ? 

Caroline,  Some  kinds  of  wood  are  heavy  certainly,  as 
oak  and  mahogany  ;  others  are  light,  as  deal  and  box. 

Emily,  I  think  I  should  call  wood  in  general  a  heavy 
body,  for  deal  and  box  are  light  only  in  comparison  to  wood 
of  a  heavier  description.  I  am  at  a  loss  to  determine  whether 
chalk  should  be  ranked  as  a  heavy  or  alight  body  ;  I  should 
be  inclined  to  say  the  former,  if  it  was  not  that  it  is  Hghter 
than  most  other  minerals.  I  perceive  that  we  have  but  vague 
notions  of  light  and  heavy.  I  wish  there  was  some  standard 
of  comparison,  to  which  we  could  refer  the  weight  of  all  other 
bodies. 

Mrs,  B,  The  necessity  of  such  a  standard  has  been  so 
much  felt,  that  a  body  has  been  fixed  upon  for  this  purpose. 
What  substance  do  you  think  would  be  best  calculated  to 
answer  this  end  ? 

Caroline,  It  must  be  one  generally  known  and  easily 
obtained,  lead  or  iron  for  instance. 

Mrs,  B,  All  the  metals  expand  by  heat,  and  condense  by 
cold.  A  piece  of  lead,  let  us  say  a  cubic  inch  for  instance, 
w^ould  have  less  specific  gravity  in  summer  than  in  winter ; 
for  it  would  be  more  dense  in  the  latter  season. 

Caroline,  But,  Mrs.  B.,  if  you  compare  the  weight  of 
equal  quantities  of  different  bodies,  they  will  all  be  alike.  You 
know  the  old  saying,  that  a  pound  of  feathers  is  as  heavy  as  a 
pound  of  lead. 

Mrs,  B,  When  therefore  we  compare  the  weight  of  differ- 
ent kinds  of  bodies,  it  w^ould  be  absurd  to  take  quantities  of 
equal  weight,  we  must  take  quantities  of  equal  bulk  ;  pints  or 
quarts,  not  ounces  or  pounds. 

Caroline,  Very  true  ;  I  perplexed  myself  by  thinking 
that  quantity  referred  to  weight,  rather  than  to  measure.  It 
is  true,  it  would  be  as  absurd  to  compare  bodies  of  the  same 
size  in  order  to  ascertain  which  was  largest,  as  to  compare 
bodies  of  the  same  weight  in  order  to  discover  which  was 
heaviest. 

Mrs.  B,  In  estimating  the  specific  gravity  of  bodies, 
therefore,  we  must  compare  equal  bulks,  and  we  shall  find 


MECHANICAL  PROPERTIES  OP  FLUIDS.  133 

that  their  specific  gravity  will  be  proportional  to  their  weights. 
The  body  which  has  been  adopted  as  a  standard  of  refeience 
is  distilled  water. 

Emily*  I  am  surprised  that  a  fluid  should  have  been  chos- 
en for  this  purpose,  as  it  must  necessarily  be  contained  in 
some  vesseij  and  the  weight  of  the  vessel  will  require  to  be 
deducted. 

Mrs,  B,  In  order  to  learn  the  specific  gravity  of  a  solid 
body,  it  is  not  necessary  to  put  a  certain  measure  of  it  in  one 
scale,  and  an  equal  measure  of  water  into  the  other  scale  : 
but  simply  to  weigh  the  body  under  trial  in  water.  If  you 
weigh  a  piece  of  gold  in  a  glass  of  water,  will  not  the  gold 
displace  just  as  much  water,  as  is  equal  to  its  own  bulk  ? 

Caroline.  Certainly,  where  one  body  is,  another  cannot 
be  at  the  same  time  ;  so  that  a  sufficient  quantity  of  water 
must  be  removed,  in  order  to  make  way  for  the  gold. 

Mrs,  B.  Yes,  a  cubic  inch  of  water  to  make  room  for  a 
cubic  inch  of  gold  ;  remember  that  the  bulk  alone  is  to  be 
considered,  the  weight  has  nothing  to  do  with  the  quantity  of 
water  displaced,  for  an  inch  of  gold  does  not  occupy  more 
space,  and  therefore  will  not  displace  more  water  than  an 
inch  of  ivory,  or  any  other  substance,  that  will  sink  in  water. 

Well,  you  will  perhaps  be  surprised  to  hear  that  the  gold 
will  weigh  less  in  water,  than  it  did  out  of  it. 

Emily.     And  for  what  reason  ? 

Mr5.  B.  On  account  of  the  upward  pressure  of  the  par- 
ticles of  water,  which  in  some  measure  supports  the  gold, 
and  by  so  doing,  diminishes  its  weight.  If  the  body  immers- 
ed in  water  was  of  the  same  weight  as  that  fluid,  it  w^ould  be 
wholly  supported  by  it,  just  as  the  water  which  it  displaces 
was  supported  previous  to  its  making  way  for  the  solid  body. 
If  the  body  is  heavier  than  the  water,  it  cannot  be  wholly 
supported  by  it ;  but  the  water  will  offer  some  resistance  to 
its  descent. 

Caroline.  And  the  resistance  which  water  offers  to  the 
descent  of  heavy  bodies  immersed  in  it,  (since  it  proceeds 
from  the  upward  pressure  of  the  particles  of  the  fluid,)  must 
in  all  cases,  I  suppose,  be  the  same  ? 

Mrs.  B.  Yes  ;  the  resistance  of  the  fluid  is  proportioned 
to  the  bulk,  and  not  to  the  weight  of  the  b  dy  immersed  in  it ; 
all  bodies  of  the  same  size,  therefore,  lose  the  same  quantity 
of  their  weight  in  water.  Can  you  form  any  idea  what  this 
loss  will  be  ? 

12 


134  MECHANICAL  TROPERTIES  OF  FLUIDS. 

Emily,  I  should  think  it  would  be  equal  to  the  weight  of 
the  water  displaced  ;  for,  since  that  portion  of  the  water  was 
supported  before  the  immersion  of  the  solid  body,  an  equal 
weight  of  the  solid  body  will  be  supported. 

Mrs,  B,  You  are  perfectly  right :  a  body  weighed  in 
water  loses  just  as  much  of  its  weight,  as  is  equal  to  that  of 
the  water  it  displaces  ;  so  that  if  you  were  to  put  the  water 
displaced  into  the  scale  to  which  the  body  is  suspended,  it 
would  restore  the  balance. 

You  must  observe,  that  when  you  weigh  a  body  in  water, 
in  order  to  ascertain  its  specific  gravity,  you  must  not  sink  the 
bason  of  the  balance  in  the  water  ;  but  either  suspend  the 
body  to  a  hook  at  the  bottom  of  the  bason,  or  else  take  off  the 
bason,  and  suspend  it  to  the  arm  of  the  balance,  (fig.  7.) 
jNow  suppose  that  a  cubic  inch  of  gold  weighed  19  ounces 
out  of  water,  and  lost  one  ounce  of  its  weight  by  being  weigh- 
ed in  water,  what  would  be  its  specific  gravity  ? 

Caroline,  The  cubic  inch  of  water  it  displaced  must 
weigh  that  .one  ounce ;  and  as  a  cubic  inch  of  gold  weighs  19 
ounces,  gold  is  19  times  as  heavy  as  water. 

Emily,  I  recollect  having  seen  a  table  of  the  comparative 
weights  of  bodies,  in  which  gold  appeared  to  me  to  be  estim- 
ated at  19  thousand  times  the  weight  of  water. 

Mrs,  B,  You  misunderstood  the  meaning  of  the  table. 
In  the  estimation  you  allude  to,  the  weight  of  water  was  reck- 
oned at  1000.  You  must  observe,  that  the  weight  of  a  sub- 
stance when  not  compared  to  that  of  any  other,  is  perfectly 
arbitrary  ;  and  when  water  is  adopted  as  a  standard,  we  may 
denominate  its  weight  by  any  number  we  please  ;  but  then 
the  weight  of  all  bodies  tried  by  this  standard  must  be  signifi- 
ed by  proportional  numbers. 

Caroline,  We  may  call  the  weight  of  water,  for  example, 
one,  and  then  that  of  gold  would  be  nineteen ;  or  if  we  choose 
to  call  the  weight  of  water  1000,  that  of  gold  would  be 
19,000.  In  short,  the  specific  gravity  means  how  much  more 
a  body  weighs  than  an  equal  bulk  of  water. 

Mrs,  B,  It  is  rather  the  weight  of  a  body  compared  with 
that  of  water  ;  for  the  specific  gravity  of  many  substances  is 
iess  than  that  of  water. 

Caroline,  Then  you  cannot  ascertain  the  specific  gravity 
of  such  substances  in  the  same  manner  as  that  of  gold  ;  for  a 
body  that  is  lighter  than  water  will  float  on  its  surface  with- 
out displacing  any  water. 


MECHANICAL  PROPERTIES  OF  FLUIDS.  135 

Mrs.  B,  If  a  body  were  absolutely  light,  it  is  true  that  it 
would  not  displace  a  drop  of  water,  but  the  bodies  we  are 
treating  of  have  all  some  weight,  however  small ;  and  will 
therefore,  displace  some  quantity  of  water.  If  the  body  be 
lighter  than  water,  it  will  not  sink  to  a  level  with  the  surface 
of  the  water,  and  therefore  it  will  not  displace  so  much  water 
as  is  equal  to  its  bulk  ;  but  it  will  displace  as  much  as  is  equal 
to  its  weight.  A  ship,  you  must  have  observed,  sinks  to 
some  depth  in  water,  and  the  heavier  it  is  laden  the  deeper  it 
sinks,  as  it  always  displaces  a  quantity  of  water  equal  to  its 
weight. 

Caroline,  But  you  said  just  now,  that  in  the  immersion 
of  gold,  the  bulk,  and  not  the  weight  of  body,  was  to  be 
considered. 

Mrs.  B.  That  is  the  case  with  all  substances  which  are 
heavier  than  water ;  but  since  those  which  are  lighter  do  not 
displace  so  much  as  their  own  bulk,  the  quantity  they  displace 
is  not  a  test  of  their  specific  gravit}^ 

In  order  to  obtain  the  specific  gravity  of  a  body  which  is 
lighter  than  water,  you  must  attach  to  it  a  heavy  one,  whose 
specific  gravity  is  known,  and  immerse  them  together  ;  the 
specific  gravity  of  the  lighter  body  may  then  be  easily  cal* 
culated. 

Emily.  But  are  there  not  some  bodies  which  have  exactly 
the  same  specific  gravity  as  v/ater  ? 

Mrs.  B.  Undoubtedly  ;  and  such  bodies  will  remain  at 
rest  in  v/hatever  situation  they  are  placed  in  water.  Here  is 
a  piece  of  wood  which,  by  being  impregnated  with  a  little 
sand,  is  rendered  precisely  of  the  weight  of  an  equal  b  ilk  of 
water  ;  in  whatever  part  of  this  vessel  of  water  you  place  it, 
you  will  find  that  it  will  remain  stationary. 

Caroline.  I  shall  first  put  it  at  the  bottom  ;  from  thence, 
of  course,  it  cannot  rise,  because  it  is  not  lighter  than  w^ater. 
Now  I  shall  place  it  in  the  middle  of  the  vessel ;  it  neither 
rises  nor  sinks,  because  it  is  neither  lighter  nor  heavier 
than  the  water.  Now  I  will  lay  it  on  the  surface  of  the  wa- 
ter ;  but  there  it  sinks  a  little— what  is  the  reason  of  that. 
Mrs.  B.  ? 

Mrs.  B.  Since  it  is  not  lighter  than  the  water,  it  cannot 
float  upon  its  surface  ;  since  it  is  not  heavier  than  water,  it 
cannot  sink  below  its  surface  :  it  will  sink  therefore,  only  till 
the  upper  surface  of  both  bodies  are  on  a  level,  so  that  the 
piece  of  wood  is  just  covered  with  water.     If  you  poured  a 


136  MECHANICAL  PROPERTIES  OP  FLUIDS  . 

few  drops  of  water  into  the  vessel,  (so  gently  as  not  to  increase 
their  momentum  by  giving  them  velocity)  they  would  mix 
with  the  water  at  the  surface,  and  not  sink  lower. 

Caroline.  This  must,  no  doubt,  be  the  reason  why  in 
drawing  up  a  bucket  of  water  out  of  a  well,  the  bucket  feels 
so  much  heavier  when  it  rises  above  the  surface  of  the  water 
in  the  well  ;  for  whilst  you  raise  it  in  the  water,  the  water 
within  the  bucket  being  of  the  same  specific  gravity  as  the 
water  on  the  outside,  will  be  wholly  supported  by  the  upward 
pressure  of  the  water  beneath  the  bucket,  and  consequently 
very  little  force  will  be  required  to  raise  it  ;  but  as  soon  as 
the  bucket  rises  to  the  surface  of  the  well  you  immediately 
perceive  the  increase  of  weight. 

Emili/.  And  how  do  you  ascertain  the  specific  gravity  of 
fluids  ? 

Mrs.  B,  By  means  of  an  instrument  called  an  hydrome- 
ter, which  I  will  show  you.  It  consists  of  a  thin  glass  ball 
A,  (fig.  8,  plate  XIII.)  with  a  graduated  tube  B,  and  the 
specific  gravity  of  the  liquid  is  estimated  by  the  depth  to 
which  the  instrument  sinks  in  it.  There  is  a  smaller  ball,  C, 
attached  to  the  instrument  below,  which  contains  a  little 
mercury ;  but  this  is  merely  for  the  purpose  of  equipoising 
the  instrument,  that  it  may  remain  upright  in  the  hquid  un- 
der trial. 

I  must  now  take  leave  of  you  ;  bat  there  remain  yet  m  any 
observations  to  be  aiade  on  fluids  ;  we  shall,  therefore,  re- 
sume this  subject  at  our  next  interview. 


CONVERSATION  XL 


OF  SPRINGS,  FOUNTAINS,  &c. 

Of  the  Ascent  of  Vapor  and  the  Formation  of  Clouds  / 
Of  the  Formation  and  Fall  of  Rainy  Sfc.  ;  Of  the  Form-* 
ation  of  Springs  ;  Of  Pavers  and  Lakes  ;  Of  Fountains^ 


CAROUNE. 

There  is  a  question  I  am  very  desirous  of  asking  you 
respecting  fluids,  JMrs.  B.,  which  has  often  perplexed  me. 
What  is  the  reason  that  the  great  quantity  of  rain  which  falls 
upon  the  earth  and  sinks  into  it,  does  not,  in  the  course  of 
time,  injure  its  solidity  ?  The  sun  and  the  wind,  I  know,  dry 
the  surface,  but  they  have  no  effect  on  the  interior  parts j 
where  there  must  be  a  prodigious  accumulation  of  moisture. 

Mrs,  B,  Do  you  not  know  that,  in  the  course  of  time,  all 
the  v/ater  which  sinks  into  the  ground  rises  out  of  it  again  ? 
It  is  the  same  water  which  successively  forms  seas,  rivers, 
springs,  clouds,  rain,  and  sometimes  hail,  snow,  and  ice.  If 
you  will  take  the  trouble  of  following  it  through  these  various 
changes,  you  will  understand  why  the  earth  is  not  yet  drown- 
ed by  the  quantity  of  water  which  has  fallen  upon  it  since  its 
creation  ;  and  you  will  even  be  convinced,  that  it  does  not 
contain  a  single  drop  more  water  now,  than  it  did  at  that 
period. 

Let  us  consider  how  the  clouds  were  originally  formed. 
When  the  first  rays  of  the  sun  warmed  the  surface  of  the 
earth,  the  heat,  by  separating  the  particles  of  v>^ater,  rendered 
them  lighter  than  the  air.  This,  you  know,  is  the  case  with 
steam  or  vapor.     What  then  ensues  ? 

Caroline.  When  lighter  than  the  air  it  will  naturally 
rise ;  ^nd  now  I  recollect  vour  telling  us  in  a  preceding  les- 

12* 


138  OF  SPRlN^Jb,  FOUNTAINS .  (IS^C. 

son,  that  the  heat  of  the  sun  transformed  the  particles  of 
water  into  vapor,  in  consequence  of  which  it  ascended  into 
the  atmosphere,  where  it  formed  clouds. 

Mrs.  B.  We  have  then  already  followed  water  through 
two  of  its  transformations  ;  from  water  it  becomes  vapor,  and 
from  vapor  clouds. 

Emihj,  But  since  this  watery  vapor  is  lighter  than  the 
air,  why  does  it  not  continue  to  rise  ;  and  why  does  it  unite 
again  to  form  clouds. 

Mi^s.  B.  Because  the  atmosphere  diminishes  in  density, 
as  it  is  more  distant  from  the  earth.  The  vapor  therefore 
which  the  sun  causes  to  exhale,  not  only  from  seas,  rivers,  and 
lakes,  but  likewise  from  the  moisture  on  the  land,  rises  till  it 
reaches  a  region  of  air  of  its  own  specific  gravity ;  and  there,  you 
know,  it  will  remain  stationary.  By  the  frequent  accession 
of  fresh  vapor  it  gradually  accumulates,  so  as  to  form  those 
large  bodies  of  vapor,  which  we  call  clouds  ;  and  these,  at 
length,  becoming  too  heavy  for  the  air  to  support,  they  fall 
to  the  ground. 

Caroline,  They  do  fall  to  the  ground,  certainly,  when 
it  rains  ;  but,  according  to  your  theory,  I  should  have  im- 
agined, that  when  the  clouds  became  too  heavy  for  the  region 
of  air  in  which  they  were  situated  to  support  them,  they  would 
descend  till  they  reached  a  stratum  of  air  of  their  own  weight, 
and  not  fall  to  the  earth  ;  for  as  clouds  are  formed  of 
vapor,  they  cannot  be  so  heavy  as  the  lowest  regions  of  the 
atmosphere,  otherwise  the  vapor  would  not  have  risen. 

Mrs.  B.  If  you  examine  the  manner  in  which  the  clouds 
descend,  it  will  obviate  this  objection.  In  falling,  several  of 
the  watery  particles  come  within  the  sphere  of  each  other's 
attraction,  and  unite  in  the  form  of  a  drop  of  water.  The 
vapor,  thus  transformed  into  a  shower,  is  heavier  than  any 
part  of  the  atmosphere,  and  consequently  descends  to  the 
earth. 

Caroline.     How  wonderfully  curious  ! 

Mrs.  B.  It  is  impossible  to  consider  any  part  of  nature 
attentively  without  being  struck  with  admiration  at  the  wisdom 
it  displays  ;  and  I  hope  you  will  never  contemplate  these 
wonders  without  feeling  your  heart  glow  with  admiration 
and  gratitude  towards  their  bounteous  Author.  Observe, 
that  if  the  waters  were  never  drawn  out  of  the  earth,  all  veg- 
etation would  be  destroyed  by  the  excess  of  moisture  ;  if,  on 
the  other  hand,  the  plants  were  not  nourished  and  refreshed 


OP  SPRINGS,  FOUNTAlNSj  SoC.  ISP 

by  occasional  showers,  the  drought  would  be  equally  fatal  to 
them.  If  the  clouds  constantly  remain  in  a  state  of  vapor, 
they  might,  as  you  remarked,  descend  into  a  heavier  stratum 
of  the  atmosphere,  but  could  never  fall  to  the  ground  ;  or 
were  the  power  of  attraction  more  than  sufficient  to  convert 
the  vapour  into  drops,  it  would  transform  the  cloud  into  a 
mass  of  water,  which,  instead  of  nourishing,  would  destroy 
the  produce  of  the  earth. 

Water  then  ascends  in  the  form  of  vapor,  and  descends  in 
that  of  rain,  snow,  or  hail,  all  of  which  ultimately  become 
water.  Some  of  this  falls  into  the  various  bodies  of  water  on 
the  surface  of  the  globe,  the  remainder  upon  the  land.  Of 
the  latter,  part  re-ascends  in  the  form  of  vapor,  part  is  absorb- 
ed by  the  roots  of  vegetables,  and  part  descends  into  the 
bowels  of  the  earth,  where  it  forms  springs. 

Emili/,     Is  rain  and  spring-water  then  the  same  ? 

J\hs,  B.  Yes,  originally.  The  only  difference  between 
rain  and  spring  water,  consists  in  the  foreign  particles  which 
the  latter  meets  with  and  dissolves  in  its  passage  through  the 
various  soils  it  traverses. 

Caroline.  Yet  spring  water  is  more  pleasant  to  the  taste, 
appears  more  transparent,  and,  I  should  have  supposed^ 
would  have  been  more  pure  than  rain  water. 

Mrs.  B,  No  ;  excepting  distilled  water,  rain  water  is 
the  most  pure  we  can  obtain  ;  and  it  is  its  purity  which  ren- 
ders it  insipid,  whilst  the  various  salts  and  different  ingredi- 
ents, dissolved  in  spring  water,  give  it  a  species  of  flavor, 
without  in  any  degree  affecting  its  transparency  ;  and  the 
filtration  it  undergoes  through  gravel  and  sand  in  the  bowels 
of  the  earth,  cleanses  it  from  all  foreign  matter  which  it  has 
not  the  povv^er  of  dissolving. 

When  rain  falls  on  the  surface  of  the  earth,  it  continues 
m^aking  its  way  downwards  through  the  pores  and  crevices  in 
the  ground.  When  several  drops  meet  in  their  subterrane- 
ous passage,  they  unite  and  form  a  little  rivulet  :  this,  in  its 
progress,  meets  with  other  rivulets  of  a  similar  description 
and  they  pursue  their  course  together  in  the  bowels  of  the 
earth,  till  they  are  stopped  by  some  substance  which  they 
cannot  penetrate. 

Caroline,  But  you  said  that  water  could  penetrate  even 
the  pores  of  gold,  and  they  cannot  meet  with  a  substance 
more  dense  ? 

Mrs.  B.     But  water  penetrates  the  pores   of  gold  only 


140  OF  SPRINGS,  FOUNTAINS,  &C. 

when  under  a  strong  compressive  force,  as  in  the  Florentine 
experiment  ;  now  in  its  passage  towards  the  centre  of  the 
earth,  it  is  acted  upon  by  no  other  power  than  gravit}^,  which 
is  not  sufficient  to  make  it  force  its  w^ay  even  through  a  stra- 
tum of  clay.  This  species  of  earth,  though  not  remarkably 
dense,  being  of  great  tenacity,  will  not  admit  the  particles  of 
water  to  pass.  When  w^ater  encounters  any  substance  of 
this  nature  therefore,  its  progress  is  stopped,  and  the  pressure 
of  the  accumulating  waters  forms  a  bed,  or  reservoir.  This 
will  be  more  clearly  explained  by  fig.  9.  plate  XIII.  which 
represents  a  section,  or  the  interior  of  a  hill  or  mountain. 
A,  is  a  body  of  water  such  as  I  have  described,  whicJi,  when 
filled  up  as  high  as  B,  (by  the  continual  accession  of  water  it 
receives  from  the  ducts  or  rivulets  a,  «,  a,  «,)  finds  a  passage 
out  of  the  cavity,  and,  impelled  by  gravity,  it  runs  on,  till  it 
makes  its  w^ay  out  of  the  ground  at  the  side  of  the  hill,  and 
there  forms  a  spring,  C. 

Caroline.  Gravity  impels  downwards  towards  the  centre 
of  the  earth  ;  and  the  spring  in  this  figiu'e  runs  in  a  horizon- 
tal direction. 

Mrs,  B.  Not  entirely.  There  is  some  declivity  from  the 
reservoir  to  the  spot  w^iere  the  water  issues  out  of  the  ground  ; 
and  gravity  you  know  will  bring  bodies  down  an  inclined 
plane,  as  well  as  in  a  perpendicular  direction. 

Caroline,  But  though  the  spring  may  descend  on  first 
issuing,  it  must  afterwards  rise  to  reach  the  surface  of  the 
earth  ;  and  that  is  in  direct  opposition  to  gravity. 

Mrs,  B,  A  spring  can  never  rise  above  the  level  of  the 
reservoir  whence  it  issues  ;  it  must,  therefore,  find  a  passage 
to  some  part  of  the  surface  of  the  earth  that  is  lower  or  nearer 
the  centre  than  the  reservoir.  It  is  true  that,  in  this  figure, 
the  spring  rises  in  its  passage  from  B  to  C  occasionally  ;  but 
this,  I  think,  with  a  little  reflection,  you  will  be  able  to  ac- 
count for. 

Emily,  Oh  yes  ;  it  is  owing  to  the  pressure  of  fluids  up- 
wards, and  the  water  rises  in  the  duct  upon  the  same  princi- 
ple as  it  rises  in  the  spout  of  a  tea-pot ;  that  is  to  say,  in  order 
to  preserve  an  equilibrium  with  the  water  in  the  reservoir. 
Now  I  think  I  understand  the  nature  of  springs  ;  the  water 
will  flow  through  a  duct,  whether  ascending  or  descending, 
provided  it  never  rises  higher  than  the  reservoir. 

Mas.  B.  Water  may  thus  be  conveyed  to  every  part  of  a 
town,  and  to  the  upper  part  of  the  houses,  if  it  is  originally 


,Fu;   J. 


TLAT£    XDT. 
Fiq.    2. 


J 

*;■:• 

B 

'■\^ 

OF  SPRINGS,  FOLNTAINSj&C.  141 

brought  from  a  height  superior  to  any  to  which  it  is  convey- 
ed. Have  you  never  observed,  when  the  pavement  of  the 
streets  liave  been  mending,  the  pipes  which  serve  as  ducts  for 
the  conveyance  of  the  water  through  the  town  ? 

Emily,  Yes,  frequently  ;  and  I  have  remarked  that  when 
any  of  these  pipes  have  been  opened,  the  water  rushes  up- 
wards from  them  with  great  velocity,  which  I  suppose  pro- 
ceeds from  the  pressure  of  the  water  in  the  reservoir,  which 
forces  it  out. 

Caroline.  I  recollect  having  once  seen  a  very  curious 
glass,  called  Tantalus's  cup  ;  it  consists  of  a  goblet,  contain- 
ing a  small  figure  of  a  man,  and  whatever  quantity  of  water 
you  pour  into  the  goblet,  it  never  rises  higher  than  the  breast 
of  the  figure.     Do  you  know  how  that  is  contrived  ? 

Mrs,  B.  It  is  by  means  of  a  syphon,  or  bent  tube,  which 
is  concealed  in  the  body  of  the  figure.  It  rises  through  one 
of  the  legs  as  high  as  the  breast,  and  there  turning  descends 
through  the  other  leg,  and  from  thence  through  the  foot  of 
the  goblet  where  the  water  runs  out.  (fig.  1.  plate  XIV.) 
When  you  pour  water  into  the  glass  A,  it  must  rise  in  the 
syphon  B,  in  proportion  as  it  rises  in  the  glass  ;  and  when  the 
glass  is  filled  to  a  level  with  the  upper  part  of  the  syphon,  the 
water  will  run  out  through  the  other  leg  of  the  figure,  and  will 
continue  running  out,  as  fast  as  you  pour  it  in  ;  therefore  the 
glass  can  never  fill  any  higher. 

Emilif,  I  think  the  new  well  that  has  been  made  at  our 
country-house,  must  be  of  that  nature.  We  had  a  great  scar- 
city of  water,  and  my  father  has  been  at  considerable  expense 
to  dig  a  well ;  after  penetrating  to  a  great  depth  before  water 
could  be  found,  a  spring  was  at  length  discovered,  but  the 
water  rose  only  a  few  feet  above  the  bottom  of  the  well  ;  and 
sometimes  it  is  quite  dry. 

Mrs.  B.  This  has,  however,  no  analogy  to  Tantalus's 
cup,  but  is  owing  to  the  very  elevated  situation  of  your 
country-house. 

Emily,  I  believe  I  guess  the  reason.  There  cannot  be  a 
reservoir  of  water  near  the  summit  of  a  hill ;  as  in  such  a  situ- 
ation there  will  not  be  a  sufficient  number  of  rivulets  formed 
to  supply  one  ;  and  without  a  reservoir,  there  can  be  no 
spring.  In  such  situations,  therefore,  it  is  necessary  to  dig^ 
very  deep,  in  order  to  meet  with  a  spring  ;  and  when  we 
give  it  vent,  it  can  rise  only  as  high  as  the  reservoir  from 


142  OF  SPRINGS  J  FOUNTAINS,  &G. 

whence  it  flows,  which  will  be  but  little,  as  the  reservoir  must 
be  situated  at  some  considerable  depth  below  the  summit  of 
the  hill. 

Caroline.  Your  explanation  appears  very  clear  and  satis- 
factory. But  I  can  contradict  it  from  experience.  At  the 
very  top  of  a  hill,  near  our  country-house,  there  is  a  large 
pond,  and,  according  to  your  theory,  it  would  be  impossible 
there  should  be  springs  in  such  a  situation  to  supply  it  with 
water.  Then  you  know  that  I  have  crossed  the  Alps,  and  I 
can  assure  you,  that  there  is  a  fine  lake  on  the  summit  of 
Mount  Cenis,  the  highest  mountain  we  passed  over. 

Mrs,  B.  Were  there  a  lake  on  the  summit  of  Mount 
Blanc,  which  is  the  highest  of  the  Alps,  it  would  indeed  be 
wonderful.  But  that  on  Mount  Cenis,  is  not  at  ail  contradic-* 
tory  to  our  theory  of  springs  ;  for  this  mountain  is  surrounded 
by  others  much  more  elevated,  and  the  springs  which  feed 
the  lake  must  descend  from  reservoirs  of  water  formed  in 
those  mountains.  This  must  also  be  the  case  with  the 
pond  on  the  top  of  the  hill :  there  is  doubtless  some  more 
considerable  hill  in  the  neighborhood  which  supplies  it  with 
water. 

Emily,  I  comprehend  perfectly,  why  the  water  in  our 
well  never  rises  high  :  but  I  do  not  understand  why  it  should 
occasionally  be  dr3\ 

Mrs,  B.  Because  the  reservoir  from  which  it  flows  being 
in  an  elevated  situation,  is  but  scantily  supplied  with  water ; 
after  a  long  drought,  therefore,  it  may  be  drained,  and  the 
spring  dry,  till  the  reservoir  be  replenished  by  fresh  rains.  It 
is  not  uncornxUion  to  see  springs  flow  with  great  violence  in 
wet  weather,  and  at  other  times  be  perfectly  dry. 

Caroline,  But  there  is  a  spring  in  our  grounds  which 
more  frequently  flows  in  dry  than  in  wet  weather :  how  is 
that  to  be  accounted  for  ? 

Mrs,  B,  The  spring  probably  comes  from  a  reservoir 
at  a  great  distance,  and  situated  very  deep  in  the  ground  : 
it  is,  therefore,  some  length  of  time  before  the  rain  reaches 
the  reservoir,  and  another  considerable  portion  must  elapse, 
whilst  the  water  is  making  its  way  from  the  reservoir  to  the 
surface  of  the  earth  ;  so  that  the  dry  weather  may  probably 
have  succeeded  the  rains  before  the  spring  begins  to  flow,  and 
the  reservoir  may  be  exhausted  by  the  time  the  wet  weather 
sets  in  aG*ain. 


OF  SPRINGS,  FOUNTAINS,  &;C,  143 

Caroline.  I  doubt  not  but  this  is  the  case,  as  the  spring 
is  in  a  very  low  situation,  therefore  the  reservoir  may  be  at  a 
great  distance  from  it. 

Mrs,  B*  Springs  which  do  not  constantly  flow,  are  called 
intermitting,  and  are  occasioned  by  the  reservoir  being  im- 
perfectly supplied.  Independently  of  the  situation,  this  is 
always  the  case  when  the  duct  or  ducts  which  convey  the 
water  into  the  reservoir  are  smaller  than  those  which  carry 
it  off. 

Caroline .  If  it  runs  out  faster  than  it  runs  in,  it  will  of 
course  sometimes  be  empty.  And  do  not  rivers  also  derive 
their  source  from  springs  ? 

Mrs.  B.  Yes,  they  generally  take  their  source  in  moun- 
tainous countries,  where  springs  are  most  abundant. 

Caroline.  I  understood  you  that  springs  were  more  rare 
in  elevated  situations. 

Mrs.  B.  You  do  not  consider  that  mountainous  countries 
abound  equally  with  high  and  low  situations.  Reservoirs  of 
water,  which  are  formed  in  the  bosom  of  mountains,  generally 
find  a  vent  either  on  their  declivity,  or  in  the  valley  beneath  ; 
while  subterraneous  reservoirs  formed  in  a  plain,  can  seldom 
find  a  passage  to  the  surface  of  the  earth,  but  remain  con- 
cealed, unless  discovered  by  digging  a  well.  When  a  spring 
once  issues  at  the  surface  of  the  earth  it  continues  its  course 
externally,  seeking  always  a  lower  ground,  for  it  can  no 
longer  rise. 

Emily.  Then  what  is  the  consequence,  if  the  spring,  or  I 
should  now  rather  call  it  a  rivulet,  runs  into  a  situation,  which 
is  surrounded  by  higher  ground. 

Mrs.  B.  Its  course  is  stopped,  the  water  accumulates, 
and  it  forms  a  pool,  pond,  or  lake,  according  to  the  dimen- 
sions of  the  body  of  water.  The  lake  of  Geneva,  in  all 
probability,  owes  its  origin  to  the  Rhone,  which  passes  through 
it  :  if,  when  this  river  first  entered  the  valley,  which  now 
forms  the  bed  of  the  Lake,  it  found  itself  surrounded  by  high- 
er grounds,  its  waters  would  there  accumulate,  till  they  rose 
to  a  level  with  that  part  of  the  valley  where  the  Rhone  now 
continues  its  course  beyond  the  Lake,  and  from  whence  it 
flows  through  valleys,  occasionally  forming  other  small  lakes 
till  it  reaches  the  sea. 

Emily.     And  are  not  fountains  of  the  nature  of  springs  ? 

Mrs.  B.  Exactly.  A  fountain  is  conducted  perpendicu- 
larly upwards,  by  the  spout  or  adjutage  A,  through  which  it 


144  OF  SPRINGS,  FOUNTAINS,  &C. 

flows  ;  and  it  will  rise  nearly  as  high  as  the  reservoir  B,  from 
whence  it  proceeds.     (Plate  XIV.  figure  2.) 

Caroline,     Why  not  quite  as  high  ? 

Mrs.  B,  Because  it  meets  with  resistance  from  the  air  in 
its  ascent  ;  and  its  motion  is  impeded  by  friction  against  the 
spoutj  where  it  rushes  out. 

Emily.  But  if  the  tube  through  which  the  water  rises  be 
smooth,  can  there  be  any  friction  ?  especially  with  a  fluid 
whose  particles  yield  to  the  shghtest  impression. 

Mrs.  B.  Friction,  (as  we  observed  in  a  former  lesson,) 
may  be  diminished  by  polishing,  but  can  never  be  entirely 
destroyed  ;  and  though  fluids  are  less  susceptible  of  friction 
than  solid  bodies,  they  are  still  aflected  by  it.  Another  reason 
why  a  fountain  will  not  rise  so  high  as  its  reservoir,  is,  that 
as  all  the  particles  of  water  spout  from  the  tube  with  an  equal 
velocity,  and  as  the  pressure  of  the  air  upon  the  exterior 
particles  must  diminish  their  velocity,  they  will  in  some 
degree  strike  against  the  under  parts,  and  force  them  sideways, 
spreading  the  column  into  a  head,  and  rendering  it  both  wider 
and  shorter  than  it  otherwise  would  be. 

At  our  next  meeting,  we  shall  examine  the  mechanical 
properties  of  the  air,  which  being  an  elastic  fluid,  difters  in 
many  respects  from  liquids. 


CONVEHSATION  XIL 


ON  THE  MECHANICAL  PROPERTIES  OF  AIR. 

Of  the  Spring  or  Elasticity  of  the  Air  ;  Of  the  weight  of 
the  Air  ;  Experiments  icith  the  Air  Pump  ;  Of  the  Ba- 
rometer ;  Mode  of  weighing  Air ;  Specific  Gravity  of 
Air  ;  Of  Pumps ;  Description  of  the  Sucking  Pump  ; 
Description  of  the  Forcing  Pump, 


MRS.  B, 

At  our  last  meeting  we  examined  the  properties  of  fluids 
in  general,  and  more  particularly  of  such  fluids  as  are  called 
liquids. 

There  is  another  class  of  fluids,  distinguished  by  the  name 
of  aeriform  or  elastic  fluids,  the  principal  of  which  is  the  air 
we  breathe,  which  surrounds  the  earth,  and  is  called  the  at- 
mosphere. 

Emily.  There  are  then  other  kinds  of  air,  besides  the 
atmosphere  ? 

Mrs,  B,  Yes  ;  a  great  variety  ;  but  they  differ  only  in 
their  chemical,  and  not  in  their  mechanical  properties  ;  and 
as  it  is  the  latter  we  are  to  examine,  we  shall  not  at  present 
inquire  into  their  composition,  but  confine  our  attention  to  the 
mechanical  properties  of  elastic  fluids  in  general. 

Caroline,     And  from  whence  arises  this  difference  ? 

Mrs,  B,  There  is  no  attraction  of  cohesion  between  the 
particles  of  elastic  fluids  ;  so  tliat  the  expansive  power  of 
heat  has  no  adversary  to  contend  with  but  gravity  ;  any 
increase  of  temperature,  therefore,  expands  elastic  fluids 
prodigiously,  and  a  diminution  proportionally  condenses 
them. 

13 


146  MECHANICAL  PROPERTIES  OF  AIR. 

The  most  essential  point  in  which  air  differs  from  other 
fluids,  is  by  its  spring  or  elasticity  ;  that  is  to  say,  its  power 
of  increasing  or  diminishing  in  bulk,  according  as  it  is  more 
or  less  compressed  :  a  power  of  which  I  have  informed  you 
liquids  are  almost  wholly  deprived. 

Emily.  I  think  I  understand  the  elasticity  of  the  air  very 
well  from  what  you  formerly  said  of  it  ;  (see  p.  33.)  but  what 
perplexes  me  is,  its  having  gravity  ;  if  it  is  heavy  and  we  are 
surrounded  by  it,  why  do  we  not  feel  its  weight  ? 

Caroline.  It  must  be  impossible  to  be  sensible  of  the 
weight  of  such  infinitely  small  particles,  as  those  of  which  the 
air  is  composed  :  particles  which  are  too  small  to  be  seen, 
must  be  too  light  to  be  felt. 

Mrs.  B.  You  are  mistaken,  my  dear  ;  the  air  is  much 
heavier  than  you  imagine  ;  it  is  true,  that  the  particles  which 
compose  it  are  small  ;  but  then,  reflect  on  their  quantity : 
the  atmosphere  extends  to  about  the  distance  of  45  miles  from 
the  earth  ;  and  its  gravity  is  such,  that  a  man  of  middling 
stature  is  computed  (when  the  air  is  heaviest)  to  sustain  the 
weight  of  about  14  tons. 

Caroline.  Is  it  possible  !  I  sliould  have  thought  such  a 
weight  would  have  crushed  any  one  to  atoms. 

Mrs.  B.  That  would,  indeed,  be  the  case,  if  it  were  not  for 
the  equality  of  the  pressure  on  every  part  of  the  body  ;  but 
when  thus  diffused  we  can  bear  even  a  much  greater  weight, 
without  any  considerable  inconvenience.  In  bathing  we 
support  the  weight  and  pressure  of  the  water,  in  addition  to 
that  of  the  atmosphere  ;  but  because  this  pressure  is  equally 
distributed  over  the  body,  we  are  scarcely  sensible  of  it ; 
whilst  if  your  shoulders,  your  head,  or  any  particular  part  of 
your  frame  were  loaded  with  the  additional  weight  of  a  hun- 
dred pounds  you  would  soon  sink  under  the  fatigue.  Be- 
sides this,  our  bodies  contain  air,  the  spring  of  which  counter- 
balances the  weight  of  the  external  air,  and  renders  us  less 
sensible  of  its  pressure. 

Caroline.  But  if  it  were  possible  to  relieve  me  from  the 
weight  of  the  atmosphere,  should  I  not  feel  more  light  and 
agile  ? 

Mrs.  B,  On  the  contrary,  the  air  within  you  meeting 
with  no  external  pressure  to  restrain  its  elasticity,  would  dis- 
tend your  body,  and  at  length  bursting  the  parts  which  con- 
nned  it,  put  a  period  to  your  existence. 


:^1ECHAN1CAL  PROPERTrES  OF  AHl.  1^3 

pump.  A  few  strokes  of  the  handle  totally  excludes  the  air 
from  the  body  of  the  pump,  and  fills  it  with  water,  which, 
having  passed  through  both  the  valves,  runs  out  at  the  spout. 

Cai^oline.  I  understand  this  perfectly.  When  the  piston 
is  elevated,  the  air  and  the  water  successively  rise  in  the 
pump  ;  for  the  same  reason  as  the  mercury  rises  in  the  ba- 
rometer. 

Emily,  I  thought  that  water  was  drawn  up  into  a  pump, 
by  suction,  in  the  same  manner  as  water  may  be  sucked 
through  a  straw. 

Mi^s.  B.  It  is  so,  into  the  body  of  the  pump  ;  for  the 
power  of  suction  is  no  other  than  that  of  producing  a  vacuum 
over  one  part  of  the  liquid,  into  which  vacuum  the  liquid  is 
forced,  by  the  pressure  of  the  atmosphere  on  another  part. 
The  action  of  sucking  through  a  straw,  consists  in  drawing  in 
and  confining  the  breath,  so  as  to  produce  a  vacuum  in  the 
mouth  ;  in  consequence  of  which,  the  air  within  the  straw 
rushes  into  the  mouth,  and  is  followed  by  the  liquid,  into 
which  the  lower  end  of  the  straw  is  immersed.  The  princi- 
ple, you  see,  is  the  same  ;  and  the  only  difference  consists  in 
the  mode  of  producing  a  vacuum.  In  suction,  the  muscular 
powers  answer  the  purpose  of  the  piston  and  valves. 

Emily,  Water  cannot,  then,  be  raised  by  a  pump  above 
32  feet ;  for  the  pressure  of  the  atmosphere  will  not  sustain  a 
column  of  water  above  that  height. 

Mrs,  B,  I  beg  your  pardon.  It  is  true  that  there  must 
never  be  so  great  a  distance  as  32  feet  from  the  level  of  the 
water  in  the  well,  to  the  valve  in  the  piston,  otherwise  the 
water  would  not  rise  through  that  valve  ;  but  when  once  the 
water  has  passed  that  opening,  it  is  no  longer  the  pressure  of 
air  on  the  reservoir  which  makes  it  ascend  ;  it  is  raised  by 
lifting  it  up,  as  you  would  raise  it  in  a  bucket,  of  which  the 
piston  formed  the  bottom.  This  common  pump  is,  therefore, 
called  the  sucking,  or  lifting-pump,  as  it  is  constructed  on  both 
these  principles.  There  is  another  sort  of  pump,  called  the 
forcing-pump  :  it  consists  of  a  forcing  power  added  to  the 
sucking  part  of  the  pump.  This  additional  power  is  exactly 
on  the  principle  of  the  syringe  :  by  raising  the  piston  you 
draw  the  water  into  the  pump,  and  by  descending  it  you  force 
the  water  out. 

Caroline,  But  the  water  must  be  forced  out  at  the  upper 
part  of  the  pump  ;  and  I  cannot  conceive  how  that  can  be 
hue  by  descending  the  piston. 


154         MECHANICAL  PROPERTIES  OF  AIR. 

Mrs,  B.  Figure  5  plate  XIV.  will  explain  the  difficulty. 
The  large  pipe  A  B  represents  the  sucking  part  of  the  pump, 
which  differs  from  the  lifting-pump,  only  in  its  piston  P  being 
unfurnished  with  a  valve,  in  consequence  of  which  the  water 
cannot  rise  above  it.  When,  therefore,  the  piston  descends, 
it  shuts  the  valve  Y  and  forces  the  water  (which  has  no  other 
vent)  into  the  pipe  D  :  this  is  likewise  furnished  with  a  valve 
V,  which,  opening  outwards,  admits  the  water,  but  prevents 
its  return. 

The  water  is  thus  first  raised  in  the  pump,  and  then  forced 
into  the  pipe,  by  the  alternate  ascending  and  descending  mo- 
tion of  the  piston,  after  a  few  strokes  of  the  handle  to  fill  the 
pipe,  from  whence  the  water  issues  at  the  spout. 

It  is  now  time  to  conclude  our  lesson.  When  next  we  meet, 
I  shall  give  you  some  account  of  wind,  and  of  sound,  which 
will  terminate  our  observations  on  elastic  fluids. 

Caroline*  And  I  shall  run  into  the  garden,  to  have  the 
pleasure  of  pumping,  now  that  I  understand  the  construction 
of  a  pump. 

Mrs.  -B.  And,  to-morrow  I  hope  you  will  be  able  to  tell 
me;  whether  it  is  a  forcing  or  a  common  lifting  pump. 


MECHANICAL  PROPERTIES  OF  AIR.  149 

Mrs.  B.  Nothing  more  easy.  I  shall  exhaust  the  air 
trom  this  little  bottle  by  means  of  the  air-pump  :  and  having 
emptied  the  bottle  of  air,  or,  in  other  words,  produced  a  va- 
cuum within  it,  I  secure  it  by  turning  this  screw  adapted  to 
its  neck  :  we  may  now  find  the  exact  weight  of  this  bottle,  by 
putting  it  into  one  of  the  scales  of  a  balance.  It  weighs  you 
see  just  two  ounces  ;  but  when  I  turn  the  screw,  so  as  to 
admit  the  air  into  the  bottle,  the  scale  which  coYitains  it  pre- 
ponderates. 

Caroline,  No  doubt,  the  bottle  filled  with  air^  is  heavier 
than  the  bottle  void  of  air  ;  and  the  additional  weight  requir- 
ed to  bring  the  scales  again  to  a  balance,  must  be  exactly  that 
of  the  air  which  the  bottle  now  contains. 

Mrs.  B.  That  weight,  you  see,  is  almost  two  grains. 
The  dimensions  of  this  bottle  are  six  cubic  inches.  Six  cubic 
inches  of  air,  therefore,  at  the  temperature  of  this  room, 
weighs  nearly  2  grains. 

Caroline.  Why  do  you  observe  the  temperature  of  the 
room,  in  estimating  the  weight  of  the  air. 

Mrs.  B.  Because  heat  rarifies  air,  and  renders  it  lighter  ; 
therefore  the  warmer  the  air  is  which  you  weigh,  the  lighter  it 
will  be. 

If  you  should  now  be  desirous  of  knowing  the  specific 
gravity  of  this  air,  we  need  only  fill  the  same  bottle  with 
water,  and  thus  obtain  the  weight  of  an  equal  quantity  of 
water — which  you  see  is  1515  grains  ;  now  by  comparing 
the  weight  of  water  to  that  of  air  we  find  it  to  be  in  the 
proportion  of  about  800  to  1. 

I  will  show  you  another  instance  of  the  weight  of  the 
atmosphere,  which  I  think  will  please  you  :  you  know  what 
a  barometer  is  ? 

Caroline.  It  is  an  instrument  which  indicates  the  state  of 
the  weather,  by  means  of  a  tube  of  quicksilver ;  but  how,  I 
cannot  exactly  say. 

Mrs.  B.  It  is  by  showing  the  weight  of  the  atmosphere. 
The  barometer  is  an  instrument  extremely  simple  in  its 
construction  :  in  order  that  you  may  understand  it,  I  will 
show  you  how  it  is  made.  I  first  fill  a  glass  tube  A  B/(fig.  3. 
plate  XIV.)  about  three  feet  in  length,  and  open  only  at  one 
end,  with  mercury  ;  then  stopping  the  open  end  with  m}^ 
finger,  I  immerse  it  in  a  cup  C,  containing  a  little  mercury. 

Emily.  Part  of  the  mercury  which  was  in  the  tube,  I 
observe,  runs  down  into  the  cup  ;  but  why  does  not  the  whole 

13* 


1  jO  MECHANICAL  PROPERTIES  OF  AlS. 

of  it  subside  in  the  cup,  for  it  is  contrary  to  the  law  of  tht 
cquiUbrium  of  fluids,  that  the  mercury  in  the  tube  should  not 
descend  to  a  level  with  that  in  the  cup. 

Mrs.  B.  The  mercury  that  has  fallen  from  the  tube  into 
the  cup,  has  left  a  vacant  space  in  the  upper  part  of  the  tube, 
to  which  the  air  cannot  gain  access  ;  this  space  is  therefore  a 
perfect  vacuum  ;  and  consequently  the  mercury  in  the  tube  is 
relieved  from  the  pressure  of  the  atmosphere,  whilst  that  in  the 
cup  remains  exposed  to  it. 

Caroline,  Oh,  now  I  understand  it  ;  the  pressure  of  the 
air  on  die  mercury  in  the  cup  forces  it  to  rise  in  the  tube, 
where  it  sustains  no  pressure. 

Emily,  Or  rather  supports  the  mercury  in  the  tube,  and 
prevents  it  from  falling. 

Mra.  B.  That  comes  to  the  same  thing  ;  for  the  power 
that  can  support  mercury  in  a  vacuum,  would  also  make  it 
ascend  when  it  met  with  a  vacuum. 

Thus  you  see,  that  the  equilibrium  of  the  mercury  is  des- 
troyed only  to  preserve  the  general  equilibrium  of  fluids. 

Caroline,  But  this  simple  apparatus  is,  in  appearance, 
very  unlike  a  barometer. 

Mrs,  B,  It  is  all  that  is  essential  to  a  barometer.  The 
tube  and  the  cup  or  vase  are  fixed  on  a  board,  for  the  con- 
v^enicnce  of  suspending  it  ;  the  board  is  graduated  for  the 
purpose  of  ascertaining  the  height  at  which  the  mercury  stands 
in  the  tube  ;  and  the  small  moveable  metal  plate  serves  to 
show  that  height  with  greater  accuracy. 

Emily,  And  at  what  height  will  the  weight  of  the  atmos- 
phere sustain  the  mercury  ? 

Mrs,  B,  About  28  inches,  as  you  will  see  by  this  baro- 
meter ;  but  it  depends  upon  the  weight  of  the  atmosphere, 
'.vhich  varies  much  according  to  the  state  of  the  weather.  The 
oreater  the  pressure  of  the  air  on  the  mercury  in  the  cup,  the 
liigher  it  will  ascend  in  the  tube.  Now  can  you  tell  me 
whether  the  air  is  heavier  in  wet  or  dry  weather  ? 

Caroline,  Without  a  moment's  reflection,  the  air  must  be 
iieaviest  in  wet  weather.  It  is  so  depressing,  emd  makes  one 
ieel  so  heavy  ;  while  in  fine  weather,  I  feel  as  light  as  a 
feather,  and  as  brisk  as  a  bee. 

Mrs,  B,  Would  it  not  have  been  better  to  have  answered 
with  a  moment's  reflection,  Caroline  ?  It  would  have  convin- 
•edyou,  that  the  air  must  be  heaviest  in  dry  weather,  for  it  is 
*hen.  that  the  mercury  is  found  to  ri^e  in  the  tube,  and  conse- 


MECHANICAL  PROPERTIES  OF  AIR.  151 

quently  the  mercury  in  the  cup  must  be  most  pressed  by  the 
air  :  and  you  know^  that  we  estimate  the  dryness  and  fairness 
of  the  weather,  by  the  height  of  the  mercury  in  the  barometer. 

Cm^oline.  Why  then  does  the  air  feel  so  heavy  in  bad 
weather  ? 

Mrs,  B,  Because  it  is  less  salubrious  when  impregnated 
with  damp.  The  lungs  under  these  circumstances  do  not 
play  so  freely,  nor  does  the  blood  circulate  so  well :  thus 
obstructions  are  frequently  occasioned  in  the  smaller  vessels, 
from  which  arise  colds,  asthmas,  agues,  fevers,  &c. 

Emily »  Since  the  atmosphere  diminishes  in  density  in  the 
upper  regions,  is  not  the  air  more  rare  upon  a  hill  than  in  a 
plain  ;  and  does  the  barometer  indicate  this  diflerence  ? 

Mrs,  B,  Certainly.  The  hills  in  this  country  are  not  suffi- 
ciently elevated  to  produce  any  very  considerable  effect  on  the 
barometer  ;  but  this  instrument  is  so  exact  in  its  indications, 
that  it  is  used  for  the  purpose  of  measuring  the  height  of  moun- 
tains, and  of  estimating  the  elevation  of  balloons. 

Emihj,  And  is  no  inconvenience  experienced  from  the 
thinness  of  the  air  in  such  elevated  situations  ? 

Mrs,  B,  Oh,  yes  ;  frequently.  It  is  sometimes  oppres- 
sive, from  being  insufficient  for  respiration  ;  and  the  expan- 
sion which  takes  place  in  the  more  dense  air  contained  within 
the  body  is  often  painful  :  it  occasions  distension,  and  some- 
times causes  the  bursting  of  the  smaller  blood-vessels  in  the 
nose  and  ears.  Besides,  in  such  situations,  you  are  more 
exposed  both  to  heat  and  cold  ;  for  though  the  atmosphere  is 
itself  transparent,  its  lower  regions  abound  with  vapors  and 
exhalations  from  the  earth,  which  float  in  it,  and  act  in  some 
degree  as  a  covering,  which  preserves  us  equally  from  the 
intensity  of  the  sun's  rays,  and  from  the  severity  of  th^  cold. 

Caroline,  Pray,  Mrs.  B.,  is  not  the  thermometer  con- 
structed on  the  same  principles  as  the  barometer  ? 

Mrs,  B,  Not  at  all.  The  rise  and  fall  of  the  fluid  in  the 
thermometer  is  occasioned  by  the  expansive  power  of  heat, 
and  the  condensation  produced  by  cold  ;  the  air  has  no  access 
to  it.  An  explanation  of  it  would,  therefore,  be  irrelevant  to 
our  present  subject. 

Emihj,  I  have  been  reflecting,  that  since  it  is  the  weight 
cf  the  atmosphere  which  supports  the  mercury  in  the  tube  of  a 
barometer,  it  would  support  a  column  of  any  other  fluid  in  the 
^ame  manner, 


152         MECHANICAL  PROPERTIES  OF  AIR. 

Mrs,  B.  Certainly  ;  but  as  mercury  is  heavier  than  all 
other  fluids,  it  will  support  a  higher  column  of  any  other 
fluid  ;  for  two  fluids  are  in  equilibrium,  when  their  height 
varies  inversely  as  their  densities.  We  find  the  weight  of 
the  atmosphere  is  equal  to  sustaining  a  column  of  water,  for 
instance,  of  no  less  than  32  feet  above  its  level. 

Caroline.  The  weight  of  the  atmosphere,  is  then,  as 
great  as  that  of  a  body  of  water  the  depth  of  32  feet  ? 

Mrs,  B,  Precisely  ;  for  a  column  of  air  of  the  height  of 
the  atmosphere,  is  equal  to  a  column  of  water  of  32  feet,  or 
one  of  mercury  of  28  inches. 

The  common  pump  is  constructed  on  this  principle.  By 
the  act  of  pumping,  the  pressure  of  the  atmosphere  is  taken 
off  the  water,  which,  in  consequence,  rises. 

The  body  of  a  pump  consists  of  a  large  tube  or  pipe,  whose 
lower  end  is  immersed  in  the  water  which  it  is  designed  to 
raise.  A  kind  of  stopper,  called  a  piston,  is  fitted  to  this 
tube,  and  is  made  to  slide  up  and  down  it  by  means  of  a  me- 
tallic rod  fastened  to  the  centre  of  the  piston. 

Emily,  Is  it  not  similar  to  the  syringe,  or  squirt,  with 
which  you  first  draw  in,  and  then  force  out  water  ? 

Mrs,  B,  It  is  ;  but  you  know  that  we  do  not  wish  to 
force  the  water  out  of  the  pump  at  the  same  end  of  the  pipe 
at  which  we  draw  it  in.  The  intention  of  a  pump  is  to  raise 
water  from  a  spring  or  well ;  the  pipe  is  therefore  placed 
perpendicularly  over  the  water  which  enters  it  at  the  lower 
extremity,  and  it  issues  at  a  horizontal  spout  towards  the 
upper  part  of  the  pump.  The  pump,  therefore,  is  rather  a 
more  complicated  piece  of  machiner}^  than  the  syringe. 

Its  various  parts  are  delineated  in  this  figure  :  (fig.  4.  plate 
XIV.)  A  B  is  the  pipe  or  body  of  the  pump,  P  the  piston,  V 
a  valve,  or  little  door  in  the  piston,  which,  opening  upwards, 
admits  the  water  to  rise  through  it,  but  prevents  its  returning, 
and  Y  a  similar  valve  in  the  body  of  the  pump. 

When  the  pump  is  in  a  state  of  inaction,  the  two  valves 
are  closed  by  their  own  weight ;  but  when,  by  drawing  down 
the  handle  of  the  pump,  the  piston  ascends,  it  raises  a  column 
of  air  which  rested  upon  it,  and  produces  a  vacuum  between 
the  piston  and  the  lower  valve  Y,  the  air  beneath  this  valve, 
which  is  immediately  over  the  surface  of  the  water,  conse- 
quendy  expands,  and  forces  its  way  through  it ;  the  water, 
then,  relieved  fyom  the  pressure  of  the  air,  ascends  into  the 


MECHANICAL  PROPERTIES  OF  AIR*  14? 

Caroline.  This  weight  of  the  atmosphere,  then,  which 
I  was  so  apprehensive  would  crush  me,  is,  in  reahty,  essen- 
tial to  my  preservation. 

Emily.  I  once  saw  a  person  cupped,  and  was  told  that 
the  swelling  of  the  part  under  the  cup  was  produced  by  taking 
away  from  that  part  the  pressure  of  the  atmosphere  ;  but  I 
could  not  understand  how  this  pressure  produced  such  an 
effect. 

Mrs,  B.  The  air  pump  affords  us  the  means  of  making  a 
great  variety  of  interesting  experiments  on  the  weight  and 
pressure  of  the  air  :  some  of  them  you  have  already  seen. 
Do  you  not  recollect,  that  in  a  vacuum  produced  within  the 
air  pump,  substances  of  various  weights  fell  to  the  bottom  in 
the  same  time  ;  why  does  not  this  happen  in  the  atmos- 
phere ? 

Caroline,  I  remember  you  told  us  it  was  owing  to  the 
resistance  which  light  bodies  meet  with  from  the  air  during 
their  fall. 

Mrs.  B.  Or,  in  other  words,  to  the  support  which  they 
received  from  the  air,  and  which  prolonged  the  time  of  their 
fall.  Now,  if  the  air  were  destitute  of  weight,  how  could  it 
support  other  bodies,  or  retard  their  fall  ? 

i  shall  now  show  you  some  other  experiments,  which 
illustrate,  in  a  striking  manner,  both  the  weight  and  elasticity 
of  air.  I  shall  tie  a  piece  of  bladder  over  this  glass  receiver, 
which,  you  will  observe,  is  open  both  at  the  top  as  well  as 
below. 

Caroline.  Why  do  you  wet  the  bladder  first  ? 
Mrs.  B.  It  expands  by  wetting,  and  contracts  in  drying  ; 
it  is  also  more  soft  and  pliable  when  wet,  so  that  I  can  make 
it  fit  better,  and  when  dry  it  will  be  tighter.  We  must  hold 
it  to  the  fire  in  order  to  dry  ;  but  not  too  near  lest  it  should 
burst  by  sudden  contraction.  Let  us  now  fix  it  on  the  air- 
pump  and  exhaust  the  air  from  underneath  it— you  will  not  be 
alarmed  if  you  hear  a  noise  ? 

Emily,  It  was  as  loud  as  the  report  of  a  gun,  and  the 
bladder  is  burst  !  Pray  explain  how  the  air  is  concerned  in 
this  experiment. 

Mrs.  B.  It  is  the  effect  of  the  weight  of  the  atmosphere 
on  the  upper  surface  of  the  bladder,  when  t  had  taken  away 
the  air  from  the  under  surface  ;  so  that  there  was  no  longer 
any  re-action  to  counterbalance  the  pressure  of  the  atmos- 
phere on  the  receiver.     You  observed  how  the  bladder  was 


148  MECHANICAL  PROPERTIES  OF  AIR'. 

pressed  inwards  by  the  weight  of  the  external  air^  in  propor- 
tion as  I  exhausted  the  receiver :  and  before  a  complete  vacu- 
um was  formed,  the  bladder,  unable  to  sustain  the  violence  of 
the  pressure,  burst  with  the  explosion  you  have  just  heard. 

1  shall  now  show  you  an  experiment,  which  proves  the 
expansion  of  the  air,  contained  within  a  body  when  it  is  re- 
lieved from  the  pressure  of  the  external  air.  You  would  not 
imagine  that  there  was  any  air  contained  within  this  shrivel- 
led apple,  by  its  appearance  ;  but  take  notice  of  it  when 
placed  within  a  receiver,  from  which  I  shall  exhaust  the  air. 

Caroline,  How  strange,  it  grows  quite  plump,  and  looks 
like  a  fresh-gathered  apple. 

Mrs,  B,  But  as  soon  as  I  let  the  air  again  into  the  receiver, 
the  apple  you  see  returns  to  its  shrivelled  state.  When  I 
took  away  the  pressure  of  the  atmosphere  the  air  within  the 
apple  expanded  and  swelled  it  out  ;  but  the  instant  the 
atmospherical  air  was  restored,  the  expansion  of  the  internal 
air  was  checked  and  repressed,  and  the  apple  shrunk  to  hs 
former  dimensions. 

You  may  make  a  similar  experiment  with  this  little  blad- 
der, which  you  see  is  perfectly  flaccid  and  appears  to  contain 
no  air  :  in  this  state  I  shall  tie  up  the  neck  of  the  bladder,  so 
that  whatever  air  remains  within  it  may  not  escape,  and  then 
place  it  under  the  receiver.  Now  observe,  as  I  exhaust  the 
receiver,  how  the  bladder  distends  ;  this  proceeds  from  the 
great  dilatation  of  the  small  quantity  of  air  which  v/as  inclosed 
within  the  bladder  when  I  tied  it  up  ;  but  as  soon  as  I  let  the 
air  into  the  receiver,  that  which  the  bladder  contains,  con- 
denses and  shrinks  into  its  small  compass  within  the  folds  of 
the  bladder. 

Emily,  These  experiments  are  extremely  amusing,  and 
they  afford  clear  proofs  both  of  the  weight  and  elasticity  of 
the  air  ;  but  I  should  like  to  know  exactly  how  much  the  air 
weighs. 

Mrs,  B.  A  column  of  air  reaching  to  the  top  of  the  at- 
mosphere, and  whose  base  is  a  square  inch,  weighs  15lbs. 
when  the  air  is  heaviest  ;  therefore  every  square  inch  of  our 
bodies  sustains  a  weight  of  15lbs.  :  and  if  you  wish  to  know 
the  weight  of  the  whole  of  the  atmosphere,  you  must  reckon 
how  many  square  inches  there  are  on  the  surface  of  the  globe, 
and  multiply  them  by  15. 

Emili/,  But  are  there  no  means  of  ascertaining  the  weighi, 
of  a  small  quantity  of  air  ? 


CONVERSATION  XIIL 


ON  WIND  AND  SOUND. 

Of  Wind  in  General;  Of  the  Trade  Wind',  Of  the 
Periodical  Trade  Winds ;  Of  the  Aerial  Tides ;  Of 
Sounds  in  General ;  Of  Sonorous  Bodies  ;  Of  Musical 
Sounds  ;  Of  Concord  or  Harmony^  and  Melody. 


MRS.B. 

Well,  Caroline,  have  you  ascertained  what  kind  of  pump 
you  have  in  your  garden  ? 

Caroline.  I  think  it  must  be  merely  a  lifting-pump,  be- 
cause no  more  force  is  required  to  raise  the  handle  than  is 
necessary  to  lift  its  weight  ;  and  in  a  forcing-pump,  by  raising 
the  handle,  you  force  the  water  into  the  smaller  pipe,  and  the 
resistance  the  water  offers  must  require  an  exertion  of  strength 
to  overcome  it. 

Mrs.  B.  I  make  no  doubt  you  are  right  ;  for  lifting 
pumps,  being  simple  in  their  construction,  are  by  far  the  most 
common. 

I  have  promised  to  day  to  give  you  some  account  of  the 
nature  of  wind.  Wind  is  nothing  more  than  the  motion  of  a 
stream  or  current  of  air,  generally  produced  by  a  partial 
change  of  temperature  in  the  atmosphere  ;  for  when  any  one 
part  is  more  heated  than  the  rest,  that  part  is  rarefied  ;  the 
equilibrium  is  destroyed,  and  the  air  in  consequence  rises. 
When  this  happens,  there  necessarily  follows  a  motion  of  the 
surrounding  air  towards  that  part,  in  order  to  restore  it  ;  this 
spot,  therefore,  receives  winds  from  every  quarter.  Those 
who  live  to  the  north  of  it  experience  a  north  wind  ;  those  to 
the  south,  a  south  wind  : — do  you  comprehend  this  ? 

Caroline.    Perfectly.     But  what  sort  of  weather  must 


I'JV  ON^WINUANP  SOUND. 

those  people  have,  who  live  on  the  spot  where  these  winds 
meet  and  interfere  ? 

Mrs,  B.  They  have  turbulent  and  boisterous  weather, 
whirlwinds,  hurricanes,  rain,  lightning,  thunder,  &c.  This 
stormy  weather  occurs  most  frequently  in  the  torrid  zone, 
where  the  heat  is  greatest :  the  air  being  more  rarefied  there, 
than  in  any  other  part  of  the  globe,  is  lighter,  and  consequent- 
ly ascends  ;  whilst  the  air  about  the  polar  regions  is  contin- 
ually flowing  from  the  poles  to  restore  the  equilibrium. 

Caroline.  This  motion  of  the  air  would  produce  a  regular 
and  constant  north  wind  to  the  inhabitants  of  the  northern 
hemisphere ;  and  a  south  wind  to  those  of  the  southern 
hemisphere,  and  continual  storms  at  the  equator,  where  these 
two  adverse  winds  would  meet. 

Mrs,  B,  These  winds  do  not  meet,  for  they  each  change 
their  direction  before  they  reach  the  equator.  The  sun,  in 
moving  over  the  equatorial  regions  from  east  to  west,  rarefies 
the  air  as  it  passes,  and  causes  the  denser  eastern  air  to  flow 
westwards,  in  order  to  restore  the  equilibrium ;  thus  producing 
ti  regular  east  wind  about  the  equator. 

Caroline,  The  air  from  the  west,  then,  constantly  goes  to 
meet  the  sun,  and  repair  the  disturbance  which  his  beams 
have  produced  in  the  equilibrium  of  the  atmosphere.  But  I 
wonder  how  you  will  reconcile  these  various  winds,  Mrs.  B.  : 
you  first  led  me  to  suppose  there  was  a  constant  struggle  be- 
tween opposite  winds  at  the  equator  producing  storm  and 
tempest ;  but  now  I  hear  of  one  regular  invariable  wind, 
which  must  naturally  be  attended  by  calm  weather. 

Emily,  I  think  I  comprehend  it :  do  not  these  winds 
from  the  north  and  south  combine  with  the  easterly  wind 
about  the  equator,  and  form  what  are  called  the  trade-winds  ? 

J  rs.  B,  Just  so,  my  dear.  The  composition  of  the  two 
w^inds  north  and  east,  produces  a  constant  north-east  wind  ; 
and  that  of  the  two  winds  south  and  east,  produces  a  regular 
south-east  wind  :  these  winds  extend  to  about  thirty  degrees 
on  each  side  of  the  equator,  the  regions  further  distant  from  it 
experiencing  only  their  respective  north  and  south  winds. 

Caroline,  But,  Mrs.  B.,  if  the  air  is  constantly  flowing 
from  the  poles  to  the  torrid  zone,  there  must  be  a  deficiency 
of  air  in  the  polar  regions  ? 

Mrs,  B,  The  light  air  about  the  equator,  which  expands 
and  rises  into  the  upper  regions  of  the  atmosphere,  ultimately 
flows  from  thence  back  to  the  poles,  to  restore  the  equilibri- 


w>N  WIND  AND  SOUND.  157' 

um  :  if  it  were  not  for  this  resource^  the  polar  atmospheric 
regions  would  soon  be  exhausted  by  the  stream  of  air,  which, 
in  the  lower  strata  of  the  atmosphere,  they  are  constantly 
sending  towards  the  equator. 

Caroline,  There  is  then  a  sort  of  circulation  of  air  in  the 
atmosphere  ;  the  air  in  the  lower  strata  flowing  from  the 
poles  towards  the  equator,  and  in  the  upper  strata,  flowing 
back  from  the  equator  towards  the  poles. 

Mrs.  B,  Exactly.  I  can  show  you  an  example  of  this 
circulation  on  a  small  scale.  The  air  of  this  room  being  more 
rarefied  than  the  external  air,  a  wind  or  current  of  air  is  pour- 
ing in  from  the  crevices  of  the  windows  and  doors,  to  restore 
the  equilibrium  ;  but  the  light  air  with  which  the  room  is 
filled  must  find  some  vent,  in  order  to  make  way  for  the 
heavy  air  which  enters.  If  you  set  the  door  a-jar,  and  hold 
a  candle  near  the  upper  part  of  it,  you  will  find  that  the  flame 
will  be  blown  outwards,  showing  that  there  is  a  current  of  air 
flowing  out  from  the  upper  part  of  the  room.  Now  place 
the  candle  on  the  floor  close  by  the  door,  and  you  will  per- 
ceive, by  the  inclination  of  the  flame,  that  there  is  also  a  cur- 
rent of  air  setting  into  the  room. 

Caroline,  It  is  just  so  ;  the  upper  current  is  the  warm 
light  air,  which  h>  driven  out  to  make  wa}^  for  the  stream  of 
cold  dense  air  which  enters  the  room  lower  down. 

Emily,  I  have  heard,  Mrs,  B.,  that  the  periodical  winds 
are  not  so  regular  on  land  as  at  sea  :  what  is  the  reason  of 
that? 

Mrs,  B,  The  land  reflects  into  the  atmosphere  a  much 
greater  quantity  of  the  sun's  rays  than  the  water  ;  tlierefore, 
that  part  of  the  atmosphere  which  is  over  the  land,  is  more 
heated  and  rarefied  than  that  which  is  over  the  sea  :  this  occa- 
sions the  wind  to  set  in  upon  the  land,  as  v/e  find  that  it  regu- 
larly does  on  the  coast  of  Guinea,  and  other  countries  in  the 
torrid  zone. 

Emily,  I  have  heard  much  of  the  violent  tempests  occa- 
sioned by  the  breaking  up  of  the  monsoons  ;  are  not  they 
^Iso  regular  trade-winds  ? 

Mrs,  B,  They  are  called  periodical  trade-winds,  as  they 
change  their  course  every  half-year.  This  variation  is  pro- 
duced by  the  earth's  annual  course  round  the  sun,  when  the 
north  pole  is  inclined  towards  that  luminary  one  half  of  the 
year,  the  south  pole  the  other  half.  During  the  summer  of 
the  northern  hemisphere,  the  countries  of  Arabia,  Persia. 
14 


158  ON  WIND  AND  SOUND. 

India,  and  China,  are  much  heated,  and  reflect  great  quanti' 
ties  of  the  sun's  rays  into  the  atmosphere,  by  which  it  becomes 
extremely  rarefied,  and  the  equilibrium  consequently  destroy- 
ed. In  order  to  restore  it,  the  air  from  the  equatorial  south- 
ern regions,  where  it  is  colder,  (as  well  as  from  the  colder 
northern  parts,)  must  necessarily  have  a  motion  towards  those 
parts.  The  current  of  air  from  the  equatorial  regions 
produces  the  trade-winds  for  the  first  six  months,  in  all  the 
seas  between  the  heated  continent  of  Asia,  and  the  equator. 
The  other  six  months,  when  it  is  summer  in  the  southern 
hemisphere,  the  ocean  and  countries  towards  the  southern 
tropic  are  most  heated,  and  the  air  over  those  parts  most 
rarefied  :  then  the  air  about  the  equator  alters  its  course,  and 
flows  exactly  in  an  opposite  direction. 

Caroline.  This  explanation  of  the  monsoons  is  very 
curious  ;  but  what  does  their  breaking  up  mean  ? 

Mrs,  B,  It  is  the  name  given  by  sailors  to  the  shifting  of 
the  periodical  winds  ;  they  do  not  change  their  course  sud- 
denly, but  by  degrees,  as  the  sun  moves  from  one  hemisphere 
to  the  other  :  this  change  is  usually  attended  by  storms  and 
hurricanes,  very  dangerous  for  shipping  ;  so  that  those  seas 
are  seldom  navigated  at  the  season  of  the  equinox. 

Emily,  I  think  I  understand  the  winds  in  the  torrid  zone 
perfectly  well  ;  but  w^hat  is  it  that  occasions  the  great  variety 
of  winds  which  occur  in  the  temperate  zones  ?  for,  according 
to  your  theory,  there  should  be  only  north  and  south  winds  in 
those  climates. 

Mrs.  B,  Since  so  large  a  portion  of  the  atmosphere  as  is 
over  the  torrid  zone  is  in  continued  agitation,  these  agitations 
in  an  elastic  fluid,  which  yields  to  the  slightest  impression, 
must  extend  every  w^ay  to  a  great  distance  ;  the  air,  therefore, 
in  all  climates,  will  suffer  more  or  less  perturbation,  according 
to  the  situation  of  the  country,  the  position  of  mountains,  val- 
leys, and  a  variety  of  other  causes  :  hence  it  is  easy  to  conceive, 
that  almost  every  climate  must  be  liable  to  variable  winds. 

On  the  sea-shore,  there  is  almost  always  a  gentle  sea-breeze 
setting  in  on  the  land  on  a  summer's  evening,  to  restore  the 
equilibrium  which  had  been  disturbed  by  reflections  from  the 
heated  surface  of  the  shore  during  the  day  ;  and  when  night 
has  cooled  the  land,  and  condensed  the  air,  we  generally  find 
it,  towards  morning,  flowing  back  towards  the  sea. 

Caroline.  I  have  observed  that  the  wind,  which  ever 
way  it  blows,  almost  always  falls  about  sun-set. 


ON  WIND  AND  SOUND.  159 

Mrs.  B.  Because  the  rarefaction  of  air  in  the  particu- 
lar spot  which  produces  the  wind,  diminishes  as  the  sun  de- 
chnesj  and  consequently  the  velocity  of  the  wind  abates. 

Emily,  Since  the  air  is  a  gravitating  fluid,  is  it  not  affect- 
ed by  the  attraction  of  the  moon  and  the  sun,  in  the  same 
manner  as  the  waters  ? 

Mrs.  B.  Undoubtedly  ;  but  the  aerial  tides  are  as  much 
greater  than  those  of  water,  as  the  density  of  water  exceeds 
that  of  air,  which,  as  you  may  recollect,  we  found  to  be  about 
800  to  1. 

Caroline.  What  a  prodigious  protuberance  that  must 
occasion  ;  How  much  the  weight  of  such  a  column  of  air  must 
raise  the  mercury  in  the  barometer  ! 

Emily..  As  this  enormous  tide  of  air  is  drawn  up  and 
supported,  as  it  were  by  the  moon,  its  weight  and  pressure,  I 
should  suppose,  would  be  rather  diminished  than  increased  ? 

Mrs.  B.  The  weight  of  the  atmosphere  is  nehher  increas- 
ed nor  diminished  by  the  aerial  tides.  The  moon's  attraction 
augments  the  bulk  as  much  as  it  diminishes  the  weight  of  the 
column  of  air  ;  these  effects,  therefore,  counterbalancing  each 
other,  the  aerial  lldes  do  not  affect  the  barometer. 

Caroline.     I  do  not  quite  understand  that. 

Mrs,  B.  Let  us  suppose  that  the  additional  bulk  of  air  at 
high  tide  raises  the  barometer  one  inch  ;  and  on  the  other 
hand,  that  the  support  which  the  moon's  attraction  affords  the 
air  diminishes  its  weight  or  pressure,  so  as  to  occasion  the 
mercury  to  fall  one  inch  ;  under  these  circumstances  the  mer- 
cury must  remain  stationary.  Thus  you  see,  that  we  can 
never  be  sensible  of  aerial  tides  by  the  barometer,  on  account 
of  the  equahty  of  pressure  of  the  atmosphere,  whatever  be  its 
height. 

The  existence  of  aerial  tides  is  not,  however,  hypothetical ; 
it  is  proved  by  the  effect  they  produce  on  the  apparent  posi- 
tion of  the  heavenly  bodies  ;  but  this  I  cannot  explain  to  you, 
till  you  understand  the  properties  of  light. 

Emily.     And  when  shall  we  learn  them  ? 

Mrs.  B.  I  shall  first  explain  to  you  the  nature  of  sound, 
which  is  intimately  connected  with  that  of  air  ;  and  I  think  at 
our  next  meeting  we  may  enter  upon  the  subject  of  optics. 

We  have  now  considered  the  effects  produced  by  the  wide 
and  extended  agitation  of  the  air  ;  but  there  is  another  kind 
of  agitation  of  which  the  air  is  susceptible — a  sort  of  vibratory 


lOO  ON  WIND  AND  SOUND. 

trembling  motion,  which,  striking  on  the  drum  of  the  ear 
produces  sound. 

Caroline.  Is  not  sound  produced  by  solid  bodies  ?  The 
voice  of  animals,  the  ringing  of  bells,  musical  instruments,  are 
all  solid  bodies.  I  know  of  no  sound  but  that  of  the  wind 
which  is  produced  by  the  air. 

Mrs.  B.  Sound,  I  assure  you,  results  from  a  tremulous 
motion  of  the  air  ;  and  the  sonorous  bodies  you  enumerate,, 
are  merely  the  instruments  by  which  that  peculiar  species  of 
motion  is  communicated  to  the  air. 

Caroline.  What  !  when  I  ring  this  little  bell,  is  it  the  air 
that  sounds,  and  not  the  bell  ? 

Mrs.  B.  Both  the  bell  and  the  air  are  concerned  in  the 
production  of  sound.  But  sound,  strictly  speaking,  is  a  per- 
ception excited  in  the  mind  by  the  motion  of  the  air  on  the 
nerves  of  the  ear  ;  the  air,  therefore,  as  well  as  the  sonorous 
bodies  which  put  it  in  motion,  is  only  the  cause  of  sound,  the 
immediate  effect  is  produced  by  the  sense  of  hearing  :  for, 
without  this  sense,  there  would  be  no  sound. 

Emily.  I  can  with  difficulty  conceive  that.  A  person 
born  deaf,  it  is  true,  lias  no  idea  ^£  sound,  because  he  hears 
none  ;  yet  that  does  not  prevent  the  real  existence  of  sound, 
as  all  those  who  are  not  deaf  can  testify. 

Mrs.  B,  I  do  not  doubt  the  existence  of  sound  to  all  those 
who  possess  the  sense  of  hearing  ;  but  it  exists  neither  in  the 
sonorous  body  nor  in  the  air,  but  in  the  mind  of  the  person 
whose  ear  is  struck  by  the  vibratory  motion  of  the  air,  pro- 
duced by  a  sonorous  body. 

To  convince  you  that  sound  does  not  exist  in  sonorous 
bodies,  but  that  air  or  some  other  vehicle  is  necessary  to  its 
production,  endeavor  to  ring  the  little  bell,  after  I  have  sus- 
pended it  under  a  receiver  in  the  air-pump,  from  which  I 
shall  exhaust  the  air 

Caroline.  This  is  indeed  very  strange  :  though  I  agitate 
it  so  violently,  it  does  not  produce  the  least  sound. 

Mrs.  B.  By  exhausting  the  receiver,  I  have  cut  off  the 
communication  between  the  air  and  the  bell ;  the  latter, 
therefore,  cannot  impart  its  motion  to  the  air. 

Caroline.  Are  you  sure  that  it  is  not  the  glass,  whicli 
covers  the  bell,  that  prevents  our  hearing  it  ? 

Mrs.  B.  That  you  may  easily  ascertain  by  letting  the  air 
into  the  receiver,  and  then  ringing  the  belL 


ON  WIND  AND  SOUND.  lOl 

Caroline.  Very  true  :  I  can  hear  it  now  almost  as  loud  as 
if  the  glass  did  not  cover  it ;  and  I  can  no  longer  doubt  but 
that  air  is  necessary  to  the  production  of  sound. 

Mrs,  B.  Not  absolutely  necessary,  though  by  far  the  most 
common  vehicle  of  sound.  Liquids,  as  well  as  air  are  capa- 
ble of  conveying  the  vibratory  motion  of  a  sonorous  body  to 
the  organ  of  hearing  ;  as  sound  can  be  heard  under  water. 
Solid  bodies  also  convey  sound,  as  I  can  soon  convince  you 
by  a  very  simple  experiment.  I  shall  fasten  this  string  by 
the  middle  round  the  poker ;  now  raise  the  poker  from  the 
ground  by  the  two  ends  of  the  string  and  hold  one  to  each  of 
your  ears  : — I  shall  now  strike  the  poker  with  a  key,  and  you 
will  find  that  the  sound  is  conveyed  to  the  ear  by  means  of  the 
strings,  in  a  much  more  perfect  manner  than  if  it  had  no  other 
vehicle  than  the  air. 

Caroline.  That  it  is,  certainly,  for  I  am  almost  stunned  by 
the  noise.  But  what  is  a  sonorous  body,  Mrs.  B.  ?  for  all 
bodies  are  capable  of  producing  some  kind  of  sound  by  the 
motion  they  communicate  to  the  air. 

Mrs.  B.  Those  bodies  are  called  sonorous,  which  produce 
clear,  distinct,  regular  and  durable  sounds,  such  as  a  bell,  a 
drum,  musical  strings,  wind-instruments,  &c.  They  owe  this 
property  to  their  elasticity  ;  for  an  elastic  body,  after  having 
been  struck,  not  only  returns  to  its  former  situation,  but  hav- 
ing acquired  momentum  by  its  velocity,  like  the  pendulum,  it 
springs  out  on  the  opposite  side.  If  I  draw  the  string  A  B, 
which  is  made  fast  at  both  ends  to  C,  it  will  not  only  return 
to  its  original  position,  but  proceed  onwards  to  D. 

This  is  its  first  vibration,  at  the  end  of  which  it  will  retain 
sufficient  velocity  to  bring  it  to  E,  and  back  again  to  F  which 
constitutes  its  second  vibration  ;  the  third  vibration  will  carry 
it  only  to  G  and  H,  and  so  on  till  the  resistance  of  the  air 
destroys  its  motion. 

The  vibration  of  a  sonorous  body  gives  a  tremulous  motion 
to  the  air  around  it,  very  similar  to  the  motion  communicated 
to  smooth  water  when  a  stone  is  thrown  into  it.  This  first 
produces  a  small  circular  wave  around  the  spot  in  which  the 
stone  falls  ;  the  wave  spreads,  and  gradually  communicates 
its  motion  to  the  adjacent  waters,  producing  similar  waves  to 
a  considerable  extent.  The  same  kind  of  waves  are  produced 
in  the  air  by  the  motion  of  a  sonorous  body,  but  with  this , 
difference,  that  as  air  is  an  elastic  fluid,  the  motion  does  not 
consist  of  regularly  extending  waves,  but  of  vibrations,  and 
14* 


i  62  On  wind  and  sound. 

are  composed  of  a  motion  forwards  and  backwards,  similar  to 
those  of  the  sonorous  body.  They  differ  also  in  the  one 
taking  place  in  a  plane,  the  other  in  all  directions.  The 
aerial  undulations  being  spherical. 

Emily,  But  if  the  air  moves  backwards  as  well  as  for- 
wards, how  can  its  motion  extend  so  as  to  convey  sound  to  a 
distance. 

Mrs.  B.  The  first  sphere  of  undulations  which  are  pro- 
duced immediately  around  the  sonorous  body,  by  pressing 
against  the  contiguous  air,  condenses  it.  The  condensed  air, 
though  impelled  forward  by  the  pressure,  re-acts  on  the  first 
set  of  undulations,  driving  them  back  again.  The  second  set 
of  undulations  v/hich  have  been  put  in  motion,  in  their  turn 
communicate  their  motion,  and  are  themselves  driven  back  by 
te-action.  Thus  there  is  a  succession  of  waves  in  the  air, 
corresponding  with  the  succession  of  waves  in  the  water. 

Caroline.  The  vibrations  of  sound  must  extend  much 
further  than  the  circular  weaves  in  water,  since  sound  is  con- 
veyed to  a  great  distance. 

Mrs.  B.  The  air  is  a  fluid  so  much  less  dense  than  water, 
that  motion  is  more  easily  communicated  to  it.  The  report 
of  a  cannon  produces  vibrations  of  the  air  which  extend  to 
several  miles  around. 

Emily.  Distant  sound  takes  some  time  to  reach  us,  since 
it  is  produced  at  the  moment  the  cannon  is  fired  ;  and  we  see 
the  light  of  the  flash  long  before  we  hear  the  report. 

Mrs.  B.  The  air  is  immediately  put  in  motion  by  the 
firing  of  a  cannon  ;  but  it  requires  time  for  the  vibrations  to 
extend  to  any  distant  spot.  The  velocity  of  sound  is  computed 
to  be  at  the  rate  of  1142  feet  in  a  second. 

Caroline.  With  what  astonishing  rapidity  the  vibrations 
must  be  communicated  !  But  the  velocity  of  sound  varies,  I 
suppose,  with  that  of  the  air  which  conveys  it.  If  the  wind 
sets  towards  us  from  the  cannon,  we  must  hear  the  report 
sooner  than  if  it  set  the  other  way. 

Mrs.  B.  The  direction  of  the  wind  makes  less  difference 
in  the  velocity  of  sound  than  you  would  imagine.  If  the  wind 
sets  from  us,  it  bears  most  of  the  aerial  waves  away  and  ren- 
ders the  sound  fainter  ;  but  it  is  not  very  considerably  longer 
in  reaching  the  ear  than  if  the  wind  blew  towards  us.  This 
Tmiform  velocity  of  sound  enables  us  to  determine  the  distance 
of  the  object  from  which  it  proceeds  ;  as  that  of  a  vessel  at 
sea  firing  a  cannon;  or  that  of  a  thunder  cloud.     If  we  do  not 


ON  WIND  AND  SOUND.  l6S 

hear  the  thunder  till  half  a  minute  after  we  see  the  lightning, 
we  conclude  the  cloud  to  be  at  the  distance  of  six  miles  and  a 
half. 

Emily.     Pray  how  is  the  sound  of  an  echo  produced  ? 

Mrs,  B.  When  the  aerial  vibrations  meet  with  an  obsta- 
cle, having  a  hard  and  regular  surface^  such  as  a  wall,  or  rock^ 
they  are  reflected  back  to  the  ear,  and  produce  the  same  sound 
a  second  time  ;  but  the  sound  will  then  appear  to  proceed 
from  the  object  by  which  it  is  reflected.  If  the  vibrations  fall 
perpendicularly  on  the  obstacle,  they  are  reflected  back  in  the 
same  line  ;  if  obliquely,  the  sound  returns  obliquely  in  the 
opposite  direction,  the  angle  of  reflection  being  equal  to  the 
angle  of  incidence. 

Caroline,  Oh,  then,  Emily,  I  now  understand  why  the 
echo  of  my  voice  behind  our  house  is  heard  so  much  plainer 
by  you  than  it  is  by  me,  when  we  stand  at  the  opposite  ends 
of  the  gravel  walk.  My  voice,  or  rather,  I  should  say,  the 
vibrations  of  air  it  occasions,  fall  obliquely  on  the  wall  of  the 
house,  and  are  reflected  by  it  to  the  opposite  end  of  the  gravel 
walk. 

Emily,  Very  true  ;  and  we  have  observed  that  when  we 
stand  in  the  middle  of  the  walk,  opposite  the  house,  the  echo 
returns  to  the  person  who  spoke. 

Mrs,  B,  Speaking-trumpets  are  constructed  on  the  princi- 
ple of  the  reflection  of  sound.  The  voice,  instead  of  being 
diflused  in  the  open  air,  is  confined  within  the  trumpet  ;  and 
the  vibrations  which  spread  and  fall  against  the  sides  of  the 
instrument,  are  reflected  according  to  the  angle  of  incidence, 
and  fall  into  the  direction  of  the  vibrations  which  proceed 
straight  forwards.  The  whole  of  the  vibrations  are  thus 
collected  into  a  focus ;  and  if  the  ear  be  situated  in  or  near  that 
spot,  the  sound  is  prodigiously  increased.  Figure  7.  plate 
XIV.  will  give  you  a  clearer  idea  of  the  speaking-trumpet : 
the  reflected  rays  are  distinguished  from  those  of  incidence, 
by  being  dotted  ;  and  they  are  brought  to  a  focus  at  F.  The 
trumpet  used  by  deaf  persons  acts  on  the  same  principle ; 
but  as  the  voice  enters  the  trumpet  at  the  large  instead  of  the 
small  end  of  the  instrument,  it  is  not  so  much  confined,  nor 
the  sound  so  much  increased. 

Emily,  Are  the  trumpets  used  as  musical  instruments 
also  constructed  on  this  principle  ? 

Mrs,  B,  So  far  as  their  form  tends  to  increase  the  sound, 
they  are  ;  but,  as  a  musical  instrument,  the  trumpet  becomes 


164  ON  WIND  AND  SOUND. 

itself  the  sonorous  body,  which  is  made  to  vibrate  by  blowing 
into  itj  and  communicates  its  vibrations  to  the  air. 

I  will  attempt  to  give  you  in  a  few  words,  some  notion  of  the 
nature  of  musical  sounds,  which  as  you  are  fond  of  music  must 
be  interesting  to  you. 

If  a  sonorous  body  be  struck  in  such  a  manner,  that  its 
vibrations  are  all  performed  in  regular  tim.es,  the  vibrations 
of  the  air  will  correspond  with  them  ;  and  striking  in  the 
same  regular  manner  on  the  drum  of  the  ear,  will  produce  the 
same  uniform  sensation  on  the  auditory  nerve  and  excite  the 
same  uniform  idea  in  the  mind  ;  or,  in  other  words,  we  shall 
hear  one  musical  tone. 

But  if  the  vibrations  of  the  sonorous  body  are  irregular, 
there  will  necessarily  follow  a  confusion  of  aerial  vibrations  ; 
for  a  second  vibration  may  commence  before  the  first  is 
finished,  meet  it  half  way  on  its  return,  interrupt  it  in  its 
course,  and  produce  harsh  jarring  sounds  which  are  called 
discords, 

Emily.  But  each  set  of  these  irregular  vibrations,  if  repeat- 
ed at  equal  intervals,  would,  I  suppose,  produce  a  musical 
tone  ?  It  is  only  their  irregular  succession  which  makes  them 
interfere,  and  occasions  discord. 

Mrs,  B,  Certainly.  The  quicker  a  sonorous  body  vibrates, 
the  more  acute^  or  sharp,  is  the  sound  produced. 

Caroline,  But  if  I  strike  any  one  note  of  the  piano-forte 
repeatedly,  whether  quickly  or  slowly,  it  always  gives  the 
same  tone. 

Mrs,  B,  Because  the  vibrations  of  the  same  string,  at  the 
same  degree  of  tension,  are  always  of  a  similar  duration.  The 
quickness  or  slowness  of  the  "vibrations  relate  to  the  single 
tones,  not  to  the  various  sounds  which  they  may  compose  by 
succeeding  each  other.  Striking  the  note  in  quick  succession, 
produces  a  more  frequent  repetition  of  the  tone,  but  does  not 
increase  the  velocity  of  the  vibrations  of  the  string. 

The  duration  of  the  vibrations  of  strings  or  chords  depends 
upon  their  length,  their  thickness  or  weight,  and  their  degree 
of  tension  :  thus,  you  find,  the  low  bass  notes  are  produced 
by  long,  thick,  loose  strings ;  and  the  high  treble  notes  by 
short,  small,  and  tight  strings. 

Caroline,  Then  the  different  length  and  size  of  the  strings 
of  musical  instruments,  serves  to  vary  the  duration  of  the 
vibrations,  and  consequently,  the  acuteness  of  gravity  of  the 
notes  ? 


ON  WIND  AND  SOUND.  l65 

Mrs,  B.  Yes.  Among  the  variety  of  tones,  there  are 
some  whichj  sounded  together,  please  the  ear,  producing  what 
we  call  harmony,  or  concord.  This  arises  from  the  agree- 
ment of  the  vibrations  of  the  two  sonorous  bodies  ;  so  that 
some  of  the  vibrations  of  each  strike  upon  the  ear  at  the  same 
time.  Thus,  if  the  vibrations  of  two  strings  are  performed  in 
equal  times,  the  same  tone  is  produced  by  both,  and  they  are 
said  to  be  in  unison. 

Emily,  Now,  then,  I  understand  why,  when  I  tune  my 
harp  in  unison  with  the  piano-forte,  I  draw  the  strings  tighter 
if  it  is  too  low,  or  loosen  them  if  it  is  at  too  high  a  pitch  ;  it  is  in 
order  to  bring  them  to  vibrate,  in  equal  times,  with  the  strings 
of  the  piano-forte. 

Mrs,  B,  But  concord,  you  know,  is  not  confined  to 
unison  ;  for  two  different  tones  harmonize  in  a  variety  of 
cases.  If  the  vibrations  of  one  string  (or  sonorous  body 
whatever)  vibrate  in  double  the  time  of  another,  the  second 
vibration  of  the  latter  will  strike  upon  the  ear  at  the  same 
instant  as  the  first  vibration  of  the  former  5  and  this  is  the 
concord  of  an  octave. 

If  the  vibrations  of  two  Strings  are  as  two  to  three,  the 
second  vibration  of  the  first  corresponds  with  the  third  viura^ 
tion  of  the  latter,  producing  the  harmony  called  a  fifth. 

Caroline,  So,  then,  when  I  strike  the  key-note  with  its 
fifth,  I  hear  every  second  vibration  of  one,  and  every  third  of 
the  other  at  the  same  time  ? 

Mrs,  B,  Yes  ;  and  the  key-note  struck  with  the  fourth  is 
likewise  a  concord,  because  the  vibrations  are  as  three  to 
four.  The  vibrations  of  a  major  third  with  the  key-note,  are 
as  four  to  five  ;  and  those  of  a  minor  third,  as  five  to  six. 

There  are  other  tones  which,  though  they  cannot  be  struck 
together  without  producing  discord,  if  struck  successively, 
gives  us  the  pleasure  which  is  called  melody.  Upon  these 
general  principles  the  science  of  music  is  founded  ;  but  I  am 
not  sufficiently  acquainted  with  it  to  enter  any  further  into  it. 

We  shall  now,  therefore,  take  leave  of  the  subject  of 
sound  ;  and,  at  our  next  interview,  enter  upon  that  of  optics, 
in  which  we  shall  consider  the  nature  of  vision,  light,  an^ 
colors. 


CONVERSATION  XIV. 


ON  OPTICS. 

Of  Luminous  J  Transparent^  and  Opaque  Bodies  :  Of  the 
Radiation  of  Light  ;  Of  Shadows  ;  Of  the  Reflection  of 
Light  ;  Opaque  Bodies  seen  only  hy  Reflected  Light  ; 
Fision  explained  ;  Camera  Obscura  ;  Image  of  Objects 
on  the  Retina, 


CAROUNE. 

I  LONG  to  begin  our  lesson  to-day,  Mrs,  B.,  for  I  expect 
that  it  will  be  ver}^  entertaining. 

Mrs,  B.  Optics  is  certainly  one  of  the  most  interesting 
branches  of  Natural  Philosophy,  but  not  one  of  the  easiest  to 
understand  ;  I  must  therefore  beg  that  you  will  give  me  the 
whole  of  your  attention. 

I  shall  first  inquire,  whether  you  comprehend  the  meaning 
of  a  luminous  body^  an  opaque  body^  and  a  transparent  body. 

Caroline,  A  luminous  body  is  one  that  shines ;  an 
opaque  .... 

Mrs,  B.  Do  not  proceed  to  the  second,  until  we  have 
agreed  upon  the  definition  of  the  first.  All  bodies  that  shine 
are  not  luminous  ;  for  a  luminous  body  is  one  that  shines  by 
its  own  light,  as  the  sun,  the  fire,  a  candle,  &c.* 

Emily,  Polished  metal  then,  when  it  shines  with  so  much 
brilliancy,  is  not  a  luminous  body  ? 

Mrs,  B.  No,  for  it  would  be  dark  if  it  did  not  receive 
light  from  a  luminous  body  ;  it  belongs,  therefore,  to  the  class 

*  The  direct  light  of  the  sun  is  calculated  to  be  equal  to  that  of  6560 
candles,  placed  at  the  distance  of  one  foot  from  the  object  ;  and  that  of 
the  moon,  to  the  light  of  one  candle  at  7  J  feet  distance  ;  of  Jupiter  s.t 
1620  feet,  and  of  Venus  at  421  fee^. 


Fi^.  1. 


PZATE  :nr 


F^.  S. 


ON  OPTICS.  167 

of  opaque  or  dark  bodies,  which  comprehend  all  such  as  are 
neither  luminous  nor  will  admit  the  light  to  pass  through 
them. 

Emily.  And  transparent  bodies,  are  those  which  admit 
the  light  to  pass  through  them ;  such  as  glass  and  water  ? 

Mrs,  B.  You  are  right.  Transparent  or  pellucid  bodies, 
are  frequently  called  mediums  ;  and  the  rays  of  light  which 
pass  through  them,  are  said  to  be  transmitted  by  them. 

Light,  when  emanated  from  the  sun,  or  any  other  luminous 
body,  is  projected  forwards  in  straight  lines  in  every  possible 
direction  ;  so  that  the  luminous  body  is  not  only  the  general 
centre  from  whence  all  the  rays  proceed,  but  every  point  of 
it  may  be  considered  as  a  centre  which  radiates  light  in  every 
durection.     (fig.  1.  plate  XV.) 

Emily,  But  do  not  the  rays  which  are  projected  in  differ- 
ent directions,  and  cross  each  other,  interfere,  and  impede 
each  other's  course  ? 

Mrs,  B,  Not  at  all.  The  particles  of  light  are  so  ex- 
tremely minute,  that  they  are  never  known  to  interfere  with 
each  other.  A  ray  of  light  is  a  single  line  of  light  projected 
from  a  luminous  body ;  and  a  pencil  of  rays,  is  a  collection  of 
rays,  proceeding  from  any  one  point  of  a  luminous  body,  as 
fig.  2. 

Caroline,  Is  light  then  a  substance  composed  of  particles 
like  other  bodies  ? 

Mrs,  B,  This  is  a  disputed  point  upon  which  I  cannot 
pretend  to  decide.  In  some  respects,  light  is  obedient  to  the 
laws  which  govern  bodies  ;  in  others  it  appears  to  be  inde- 
pendent of  them:  thus  though  its  corrse  is  guided  by  the 
laws  of  motion,  it  does  not  seem  to  be  influenced  by  those  of 
gravity.  It  has  never  been  discovered  to  have  weight,  though 
a  variety  of  interesting  experiments  have  been  made  with  a 
view  of  ascertaining  that  point ;  but  we  are  so  ignorant  of 
the  intimate  nature  of  light,  that  an  attempt  to  investigate  it 
would  lead  us  into  ^  labyrinth  of  perplexity,  if  not  of  error  ; 
we  shall  therefore  confine  our  attention  to  those  properties  of 
light  which  are  well  ascertained. 

Let  us  return  to  the  examination  of  the  effects  of  the  radia- 
tion of  light  from  a  luminous  body.  Since  the  rays  of  light 
are  projected  in  straight  lines,  when  they  meet  with  an  opaque 
body  through  which  they  are  unable  to  pass,  they  are  stop- 
ped short  in  their  course  5  for  they  cannot  move  in  a  curve 
line  round  the  body. 


168  ON  OPTICS. 

Caroline,  No,  certainly  ;  for  it  would  require  some  other 
force  besides  that  of  projection,  to  produce  motion  in  a  curve 
line. 

Mrs.  B.  The  interruption  of  the  rays  of  light,  by  the 
opaque  body,  produces,  therefore,  darkness  on  the  opposite 
side  of  it ;  and  if  this  darkness  fall  upon  a  wall,  a  sheet  of 
paper,  or  any  object  whatever,  it  forms  a  shadow. 

Emily,  A  shadow  then  is  nothing  more  than  darkness 
produced  by  the  intervention  of  an  opaque  body,  which 
prevents  the  rays  of  light  from  reaching  an  object  behind  the 
opaque  body. 

Caroline,  Why  then  are  shadows  of  different  degrees  of 
darkness  ;  for  I  should  have  supposed  from  your  definition  of 
a  shadow,  that  it  would  have  been  perfectly  black  ? 

Mrs,  B,  It  frequently  happens  that  a  shadow  is  produced 
by  an  opaque  body  interrupting  the  course  of  the  rays  from 
one  luminous  body,  while  light  from  another  reaches  the 
space  where  the  shadow  is  formed,  in  wliich  case  the  shadow- 
is  proportionally  fainter.  This  happens  if  the  opaque  body 
be  lighted  by  two  candles  :  if  you  extinguish  one  of  them^ 
the  shadow  will  be  both  deeper  and  more  distinct. 

Caroline.     But  yet  it  will  not  be  perfectly  dark. 

Mrs.  B,  Because  it  is  still  slightly  illumined  by  light 
reflected  from  the  walls  of  the  room,  and  other  surrounding 
objects. 

You  must  observe,  also,  that  when  a  shadow  is  produced 
by  the  interruption  of  rays  from  a  single  luminous  body,  the 
darkness  is  proportional  to  the  intensity  of  the  light. 

Emily.  I  should  have  supposed  the  contrary  ;  for  as  the 
light  reflected  from  surrounding  objects  on  the  shadow,  must 
be  in  proportion  to  the  intensity  of  the  light,  the  stronger  the 
light,  the  more  the  shadow  will  be  illumined. 

Mrs.  B.  Your  remark  is  perfectly  just ;  but  as  we  have 
no  means  of  estimating  the  degrees  of  light  and  of  darkness 
but  by  comparison,  the  strongest  light  will  appear  to  produce 
the  deepest  shadow.  Hence  a  total  eclipse  of  the  sun  occa- 
sions a  more  sensible  darkness  than  mid-night,  as  it  is  imme- 
diately contrasted  with  the  strong  light  of  noon-day. 

Caroline.  The  re-appearance  of  the  sun  after  an  eclipse, 
must  by  the  same  contrast,  be  remarkably  brilliant. 

Mrs.  B.  Certainly.  There  are  several  things  to  be  ob- 
served in  regard  to  the  form  and  extent  of  shadows.  If  the 
luminous  body  A  (Jig.  3.)  is  larger  than  the  opaque  body  B, 


ON  OPTICS.  169 

the  shadow  will  gradually  diminish  in  size,  till  it  terminate  in 
a  point. 

Caroline.  This  is  the  case  with  the  shadows  of  the  earth 
and  the  moon,  as  the  sun  which  illumines  them,  is  larger  than 
either  of  those  bodies.  And  why  is  it  not  the  case  with  the 
shadows  of  terrestrial  objects,  which  are  equally  illumined  by 
the  sun  ?  but  their  shadows,  far  from  diminishing,  are  always 
larger  than  the  object,  and  increase  with  the  distance  from  it. 

Mrs.  B.  In  estimating  the  effect  of  shadows,  we  must 
consider  the  apparent  not  the  re«/  dimensions  of  the  lumin- 
ous body  ;  and  in  this  point  of  view,  the  sun  is  a  small  object 
compared  wdth  the  generality  of  the  terrestrial  bodies  which  it 
illumines  :  and  when  the  luminous  body  is  less  than  the 
opaque  body,  the  shadow  will  increase  with  the  distance  to 
infinity.  All  objects,  therefore,  which  are  apparently  larger 
than  the  sun,  cast  a  magnified  shadow.  This  will  be  best 
exemplified,  by  observing  the  shadow  of  an  object  lighted  by 
a  candle. 

Emily,  I  have  often  noticed,  that  the  shadow  of  my  fig- 
ure against  the  wall,  grows  larger  as  it  is  more  distant  from 
me,  which  is  owing,  no  doubt,  to  the  candle  that  shines  on 
me  being  much  smaller  than  m3^self  ? 

Mrs.  B.  Yes.  The  shadow  of  a  figure  A,  (fig.  4.)  varies 
in  size,  according  to  the  distance  of  the  several  surfaces  B  C 
D  E,  on  which  it  is  described. 

Caroline.  I  have  observed,  that  two  candles  produce  two 
shadows  from_  the  same  object  ;  whilst  it  would  appear,  from 
what  you  said,  that  they  should  rather  produce  only  half  a 
iihadow,  that  is  to  say,  a  very  faint  one. 

Mrs.  B.  Th^  number  of  lights  (in  different  directions) 
while  it  decreases  the  intensity  of  the  shadow,  increases  their 
number  which  always  corresponds  with  that  of  the  lights  ; 
for  each  light  makes  the  opaque  body  cast  a  different  shadow, 
as  illustrated  by  fig.  5.  It  represents  a  ball  A,  lighted  by 
three  candles  B,  C,  D,  and  you  observe  the  light  B  produces 
tlie  shadow  6,  the  light  C  the  shadow  c,  and  the  light  D  the 
shadow  d. 

Emily.  I  think  we  now  understand  the  nature  of  shadows 
very  well ;  but  pray  what  becomes  of  the  rays  of  light  which 
opaque  bodies  arrest  in  their  course,  and  the  interruption  of 
which  is  the  occasion  of  shadows  ? 

Mrs.  B.     Your  question  leads  to  a  very  important  property 
of  light,  Reflection,     When  rays  of  light  encounter  an  opaque 
15 


170  ON  OPTICS. 

body,  which  they  cannot  traverse,  part  of  them  are  absorbed 
by  it,  and  part  are  reflected,  and  rebound  just  as  an  elastic 
ball  which  is  struck  against  a  wall. 

Emily,  And  is  light  in  its  reflection  governed  by  the  same 
laws  as  solid  elastic  bodies  ? 

Mrs,  B,  Exactly.  If  a  ray  of  light  fall  perpendicularly 
on  an  opaque  body,  it  is  reflected  back  in  the  same  line,  to- 
wards the  point  whence  it  proceeded.  If  it  fall  obliquely,  it 
is  reflected  obliquely,  but  in  the  opposite  direction  ;  the  angle 
of  incidence  being  equal  to  the  angle  of  reflection.  You  re- 
collect that  law  in  mechanics  ? 

Emily,     Oh  yes,  perfectly. 

Mrs,  B,  If  you  will  shut  the  shutters,  we  shall  admit  a 
ray  of  the  sun's  light  through  a  very  small  aperture,  and  I 
can  show  you  how  it  is  reflected.  I  now  hold  this  mirror,  so 
that  the  ray  shall  fall  perpendicularly  upon  it. 

Caroline,  I  see  the  ray  which  falls  upon  the  mirror,  but 
not  that  which  is  reflected  by  it. 

Mrs.  B,  Because  its  reflection  is  directly  retrograde. 
The  ray  of  incidence  and  that  of  reflection  both  being  in  the 
same  line,  though  in  opposite  directions,  are  confounded 
together. 

Emily,  The  ray  then  which  appears  to  us  single,  is  really 
double,  and  is  composed  of  the  incident  ray  proceeding  to 
the  mirror,  and  of  tl^e  reflected  ray  returning  from  the 
mirror. 

Mrs,  B,  Exactly  so.  We  shall  now  separate  them  by 
holding  the  mirror  M,  (fig.  6.)  in  such  a  manner,  that  the 
incident  ray  A  B  shall  fall  obliquely  upon  it — you  see  the 
reflected  ray  B  C,  is  marching  ofl'  in  another  direction.  If 
we  draw  a  line  from  the  point  of  incidence  B,  perpendicular 
to  the  mirror,  it  will  divide  the  angle  of  incidence  from  the 
angle  of  reflection,  and  you  will  see  that  they  are  equal. 

Emily.  Exactly  ;  and  now  that  you  hold  the  mirror  so, 
that  the  ray  falls  more  obliquely  on  it,  it  is  also  reflected 
more  obliquely,  preserving  the  equality  of  the  angles  of  inci- 
dence and  reflection. 

Mrs,  B,  It  is  by  reflected  rays  only  that  we  see  opaque 
objects.  Luminous  bodies  send  rays  of  light  immediately  to 
our  eyes,  but  the  rays  which  they  send  to  other  bodies  are 
invisible  to  us,  and  are  seen  only  when  they  are  reflected  or 
transmitted  by  those  bodies  to  our  eyes. 

Emily,     But  have  we  not  just  seen  the  ray  of  light  in  it<? 


ON  OPTICS.  171 

passage  from  the  sun  to  the  mirror,  and  its  reflettion  ?  yet 
in  neither  case  were  those  rays  in  a  direction  to  enter  our 
eyes. 

Mrs,  B.  No.  What  you  saw  was  the  hght  reflected  to 
your  eyes  by  small  particles  of  dust  floating  in  the  air,  and 
on  which  the  ray  shone  in  its  passage  to  and  from  the 
mirror. 

Caroline,  Yet  I  see  the  sun  sliining  on  that  house  yonder^ 
as  clearly  as  possible. 

Mrs,  B,  Indeed  you  cannot  see  a  single  ray  which  passes 
from  the  sun  to  the  house  ;  you  see  no  rays  but  those  which 
enter  your  eyes  ;  therefore  it  is  the  rays  which  are  reflected 
by  the  house  to  you,  and  not  those  which  proceed  from  the 
sun  to  the  house,  that  are  visible  to  you. 

Caroline.  Why  then  does  one  side  of  the  house  appear 
to  be  in  sunshine,  and  the  other  in  the  shade  ?  for  if  I  cannot 
see  the  sun  shine  upon  it,  the  whole  of  the  house  should 
appear  in  the  shade. 

Mrs,  B,  That  side  of  the  house  which  the  sun  shines 
upon,  reflects  more  vivid  and  luminous  rays  than  the  side 
which  is  in  shadow,  for  the  latter  is  illumined  only  by  rays 
reflected  upon  it  by  other  objects,  these  rays  are  therefore 
twice  reflected  before  they  reach  your  sight ;  and  as  light 
is  more  or  less  absorbed  by  the  bodies  it  strikes  upon,  every 
time  a  ray  is  reflected  its  intensity  is  diminished. 

Caroline,  Still  I  cannot  reconcile  myself  to  the  idea,  that 
we  do  not  see  the  sun's  rays  shining  on  objects,  but  only  those 
which  objects  reflect  to  us. 

Mrs,  B.  I  do  not,  however,  despair  of  convincing  you  of 
it.  Look  at  that  large  sheet  of  water,  can  you  tell  why  the  sun 
appears  to  shine  on  one  part  of  it  only  ? 

Caroline,  No,  indeed  ;  for  the  whole  of  it  is  equally 
exposed  to  the  sun.  This  partial  brilliancy  of  water  has 
often  excited  my  wonder  ;  but  it  has  struck  me  more  par- 
ticularly by  moon-light.  I  have  frequently  observed  a  vivid 
streak  of  moonshine  on  the  sea,  while  the  rest  of  the  water 
remained  in  deep  obscurity,  and  yet  there  was  no  apparent 
obstacle  to  prevent  the  moon  from  shining  on  every  part  of 
the  water  equally. 

Mrs,  B,  By  moon-light  the  effect  is  more  remarkable,  on 
account  of  the  deep  obscurity  of  the  other  parts  of  the  water  ; 
while  by  the  sun's  light  the  eflect  is  too  strong  for  the  eye  to 
be  able  to  contemplate  it. 


172  «N  OPTICS. 

Caroline.  But  if  the  sun  really  shines  on  every  part  of 
that  sheet  of  water^  why  does  not  every  part  of  it  reflect  rays 
to  my  eyes  ? 

Mrs.  B,  The  reflected  rays  are  not  attracted  out  of  their 
natural  course  by  your  eyes.  The  direction  of  a  reflected 
ray,  you  know,  depends  on  that  of  the  incident  ray  ;  the  sun's 
rays,  therefore,  which  fall  with  various  degrees  of  obliquity 
upon  the  water,  are  reflected  in  directions  equally  various ; 
some  of  these  will  meet  your  eyes,  and  you  will  see  them,  but 
those  which  fall  elsewhere  are  invisible  to  you. 

Caroline.  The  streak  of  sunshine,  then,  which  we  now 
see  upon  the  water,  is  composed  of  those  rays  which  by  their 
reflection  happen  to  fall  upon  my  eyes  ? 

Mrs.  B.     Precisely. 

Emily.  But  is  that  side  of  the  house  yonder^  which  ap- 
pears to  be  in  shadow,  really  illumined  by  the  sun,  and  its 
rays  reflected  another  way  ? 

Mrs.  B.  No ;  that  is  a  diflerent  case  from  the  sheet  of 
water.  That  side  of  the  house  is  really  in  shadow  ;  it  is  the 
west  side,  which  the  sun  cannot  shine  upon  till  the  afternoon. 

Emily.  Those  objects,  then,  which  are  illumined  by 
reflected  rays,  and  those  which  receive  direct  rays  from  the 
sun,  but  which  do  not  reflect  those  rays  towards  us,  appear 
equally  in  shadow  ? 

Mrs.  B.  Certainly  ;  for  we  see  them  both  illumined,  by- 
reflected  rays.  That  part  of  the  sheet  of  water,  over  which 
the  trees  cast  a  shadow,  by  what  light  do  you  see  it. 

Emily.  Since  it  is  not  by  the  sun's  direct  rays^  it  must  be 
by  those  reflected  on  it  from  other  objects,  and  which  it  again 
reflects  to  us. 

Caroline.  But  if  we  see  all  terrestrial  objects  by  reflected 
light,  (as  we  do  the  moon,)  why  do  they  appear  so  bright  and 
luminous  ?  I  should  have  supposed  that  reflected  rays  would 
have  been  dull  and  faint,  like  those  of  the  moon. 

Mrs.  B.  The  moon  reflects  the  sun's  light  with  as  much 
vividness  as  any  terrestrial  object.  If  you  look  at  it  on  a 
clear  night,  it  will  appear  as  bright  as  a  sheet  of  water,  the 
walls  of  a  house,  or  any  object  seen  by  daylight  and  on  which 
the  sun  shines.  The  rays  of  the  moon  are  doubtless  feeble^ 
when  compared  with  those  of  the  sun;  but  that  would  not 
be  a  fair  comparison,  for  the  former  are  incident,  the  latter 
reflected  rays. 

Caroline.     True  ?  and  when  we  see  terrestrial  objects  by 


TLATE.  JOi 


ON  OPTICS.  173 

moon-light,  the  light  has  been  twice  reflected^  and  is  conse- 
quently proportionally  fainter. 

Mrs.  B.  In  traversing  the  atmosphere^  the  ra3'S,  both  of 
the  sun  and  moon,  lose  some  of  their  light.  For  though  the 
pure  air  is  a  transparent  medium^  which  transmits  the  rays  of 
light  freely^  we  have  observed,  that  near  the  surface  of  the 
earth  it  is  loaded  with  vapors  and  exhalations^  by  which  some 
portion  of  them  are  absorbed. 

Caroline,  I  have  often  noticed  that  an  object  on  the 
summit  of  a  hill  appears  more  distinct  than  one  at  an  equal 
distance  in  a  valley,  or  on  a  plain  ;  which  is  owing,  I  sup- 
pose, to  the  air  being  more  free  from  vapors  in  an  elevated 
situation,  and  the  reflected  rays  being  consequently  brighter. 

Mrs.  B.  That  may  have  some  sensible  effect  ;  but  when 
an  object  on  the  summit  of  a  hill  has  a  back  ground  of  light 
sky,  the  contrast  with  the  object  makes  its  outline  more 
distinct. 

Caroline.  I  now  feel  well  satisfied  that  we  see  opaque 
objects  only  by  reflected  rays  :  but  I  do  not  understand  how 
these  rays  show  us  the  objects  from  which  they  proceed  ? 

Mrs.  B.  The  rays  of  light  enter  at  the  pupil  of  the  eye, 
and  proceed  to  the  retina,  or  optic  nerve,  which  is  situated  at 
the  back  part  of  the  eye-ball  ;  and  there  they  describe  the 
figure,  color,  and  (excepting  size)  form  a  perfect  representa- 
tion of  the  object  from  which  they  proceed.  We  shall  again 
close  the  shutters,  and  admit  the  light  through  the  small 
aperture,  and  you  will  see  a  picture  on  the  wall,  opposite  the 
aperture,  similar  to  that  which  is  delineated  on  the  retina  of 
the  eye. 

Caroline.  Oh,  how  v/onderful  !  There  is  an  exact  picture 
in  miniature  of  the  garden,  the  gardener  at  work,  the  trees 
blown  about  by  the  wind.  The  landscape  would  be  perfect, 
if  it  were  not  reversed  ;  the  ground  being  above,  and  the  sky 
beneath. 

Mrs.  B.  It  is  not  enough  to  admire,  you  must  under- 
stand this  phenomenon,  which  is  called  a  camera  obscura^ 
from  the  necessity  of  darkening  the  room,  in  order  to  exhibit 
it. 

This  picture  is  produced  by  the  rays  of  light  reflected  from 
the  various  objects  in  the  garden,  and  which  are  admitted 
through  the  hole  in  the  window  shutter. 

The  rays  from  the  glittering  weathercock  at  the  top  of  the 
alcove  A,  (plate  XVL  fig.  1.)  represent  it  in  this  spot  a  ; 

15* 


;/  4  ON  OPTIC.-V. 

for  the  weathercock  being  much  higher  than  the  aperture  in 
the  shutter,  only  a  few  of  the  rays,  which  are  reflected  by  it 
in  an  obUquely  descending  direction,  can  find  entrance  there. 
The  rays  of  hght,you  know,  always  move  in  straight  lines  ; 
those,  therefore,  which  enter  the  room  in  a  descending  direc- 
tion, will  continue  their  course  in  the  same  direction,  and 
will,  consequently,  fall  upon  the  lower  part  of  the  wall  oppo- 
site the  aperture,  and  represent  the  weathercock  reversed  in 
that  spot,  instead  of  erect  in  the  uppermost  part  of  the  land- 
scape. 

E?nily,  And  the  rays  of  light  from  the  steps  (B)  of  the 
alcove,  in  entering  the  aperture,  ascend,  and  will  describe 
those  steps  in  the  highest  instead  of  the  lowest  part  of  the 
landscape. 

Mrs,  B.  Observe,  too,  that  the  rays  coming  from  the 
alcove,  which  is  to  our  left,  describe  it  on  the  wall  to  the 
right  ;  while  those  which  are  reflected  by  the  walnut-tree  C 
D,  to  our  right,  delineate  its  figure  in  the  picture  to  the  left 
c  ch  Thus  the  rays,  coming  in  diflerent  directions,  and 
proceeding  always  in  right  lines,  cross  each  other  at  their 
entrance  through  the  aperture  :  those  which  come  above 
proceed  below,  those  from  the  right  go  to  the  left,  those  from 
the  left  towards  the  right  ;  thus  every  object  is  represented 
in  the  picture,  as  occupying  a  situation  the  very  reverse  of 
that  which  it  does  in  nature. 

Caroline*  Excepting  the  flower-pot  E  F,  which,  though 
its  position  is  reversed,  has  not  changed  its  situation  in  the 
landscape. 

Mrs*  B.  The  flower-pot  is  directly  in  front  of  the  aper- 
ture ;  so  that  its  rays  fall  perpendicularly  upon  it,  and,  con- 
sequently, proceed  perpendicularly  to  the  wall,  w^here  they 
delineate  the  object  directly  behind  the  aperture. 

Emily,  And  is  it  thus  that  the  picture  of  objects  is  painted 
on  the  retina  of  the  ej'e  ?  * 

Mrs,  B.  Precisely.  The  pupil  of  the  eye,  through  which 
the  rays  of  light  enter,  represents  the  aperture  in  the  window- 
shutter  ;  and  the  image  delineated  on  the  retina,  is  exactly 
similar  to  the  picture  on  the  wall. 

*  Take  off  the  sclerotica  from  the  back  part  of  the  eye  of  an  ox,  or 
othei'  animal,  and  place  the  eye  in  the  hole  of  the  window -shutter  of  a 
dark  room,  with  its  fore  part  towards  the  external  objects  ;  a  person  in 
the  room  will  through  the  transparent  coat,  see  the  inverted  ima^e  paint-- 
ed  upon  the  retina. 


ON  OPTICS.  175 

Caroline.  You  do  not  mean  to  say,  that  we  see  only  the 
representation  of  the  object  which  is  painted  on  the  retina- 
and  not  the  object  itself  ? 

Mrs.  B.  Ify  by  sight,  you  understand  that  sense  by  which 
the  presence  of  objects  is  perceived  by  the  mind,  through  the 
means  of  the  eyes,  we  certainly  see  only  the  image  of  those 
objects  painted  on  the  retina. 

Caroline.     This  appears  to  me  quite  incredible. 

Mrs.  B.  The  nerves  are  the  only  part  of  our  frame  ca- 
pable of  sensation  :  they  appear,  therefore,  to  be  the  instru- 
ments which  the  mind  employs  in  its  perceptions  ;  for  a 
sensation  always  conveys  an  idea  to  the  mind.  Now  it  is 
known,  that  our  nerves  can  be  affected  only  by  contact  ;  and 
for  this  reason  the  organs  of  sense  cannot  act  at  a  distance  : 
for  instance,  we  are  capable  of  smelling  only  particles  which 
are  actually  in  contact  with  the  nerves  of  the  nose.  We 
have  already  observed,  that  the  odom'  of  a  flower  consists  m 
effluvia,  composed  of  very  minute  particles,  which  penetrate 
the  nostrils,  and  strike  upon  the  olfactory  nerves,  which 
instantly  convey  the  idea  of  smell  to  the  mind. 

Emily.  And  sound,  though  it  is  said  to  be  heard  at  a 
distance,  is,  in  fact,  heard  only  when  the  vibrations  of  the 
air,  which  convey  it  to  our  ears,  strike  upon  the  auditory- 
nerve. 

Caroline.  There  is  no  explanation  required,  to  prove 
that  the  senses  of  feeling  and  of  tasting  are  excited  only  by 
contact. 

Mrs.  B.  And  I  hope  to  convince  you,  that  the  sense  of 
sight  is  so  likewise.  The  nerves,  which  constitute  the  sense 
of  sight,  are  not  different  in  their  nature  from  those  of  the 
other  organs ;  they  are  merely  instruments  which  convey 
ideas  to  the  mind,  and  can  be  affected  only  on  contact.  Now, 
since  real  objects  cannot  be  brought  to  touch  the  optic  nerve, 
the  image  of  them  is  conveyed  thither  by  the  rays  of  light 
proceeding  from  real  objects,  which  actually  strike  upon  the 
optic  nerve,  and  form  that  image  which  the  mind  perceives. 

Caroline.  While  I  listen  to  your  reasoning,  I  feel  con- 
vinced ;  but  when  I  look  upon  the  objects  around,  and  think 
that  I  do  not  see  them,  but  merely  their  image  painted  in  my 
eyes,  my  belief  is  again  staggered.  I  cannot  reconcile  myself 
to  the  idea,  that  I  do  not  really  see  this  book  which  I  hold  in 
TTiy  hand,  nor  the  words  which  I  read  in  it. 


176  ON  OPTICS. 

Mrs.  B.  Did  it  ever  occur  to  you  as  extraordinary^  that 
you  never  beheld  your  own  face  ? 

Caroline,  No  ;  because  I  so  frequently  see  an  exact 
representation  of  it  in  the  looking-glass. 

Mrs,  B,  You  see  a  far  more  exact  representation  of  ob- 
jects on  the  retina  of  your  eye:  it  is  a  much  more  perfect 
mirror  than  any  made  by  art. 

Emily,  But  is  it  possible,  that  the  extensive  landscape^ 
which  I  now  behold  from  the  window,  should  be  represented 
on  so  small  a  space  as  the  retina  of  the  eye  ? 

Mrs,  B,  It  would  be  impossible  for  art  to  paint  so  small 
and  distinct  a  miniature  ;  but  nature  works  with  a  surer  hand, 
and  a  more  delicate  pencil.  That  power,  which  forms  the 
feathers  of  the  butterfly,  and  the  flowerets  of  the  daisy,  can 
alone  portray  so  admirable  and  perfect  a  miniature  as  that 
which  is  represented  on  the  retina  of  the  eye. 

Caroline,  But,  Mrs.  B.,  if  we  see  only  the  image  of  ob- 
jects, why  do  we  not  see  them  reversed,  as  you  showed  us 
they  were  in  the  camera  obscura  ?  Is  not  that  a  strong 
argument  against  your  theory  ? 

Mrs,  B,  Not  an  unanswerable  one,  I  hope.  The  image 
on  the  retina,  it  is  true,  is  reversed,  like  that  in  the  camera 
obscura  ;  as  the  rays,  unless  from  a  very  small  object,  inter- 
sect each  other  on  entering  the  pupil,  in  the  same  manner  as 
they  do  on  entering  the  camera  ol3scura.  The  scene,  however^ 
does  not  excite  the  idea  of  being  inverted,  because  we  always 
see  an  object  in  the  direction  of  the  rays  which  it  sends  to  us. 

Emily,     I  confess  I  do  not  understand  that. 

Mrs.  B,  It  is,  I  think,  a  diflicult  point  to  explain  clearly. 
A  ray  which  comes  from  the  upper  part  of  an  object  ;  des- 
cribes the  image  on  the  lower  part  of  the  retina  ;  but  expe- 
rience having  taught  us,  that  the  direction  of  that  ray  is  from 
above,  we  consider  that  part  of  the  object  it  represents  as 
uppermost.  The  rays  proceeding  from  the  lower  part  of  an 
object  fall  upon  the  upper  part  cflhe  retina  ;  but  as  we  know 
their  direction  to  be  from  below,  we  see  that  part  of  the  object 
they  describe  as  the  lowest. 

Caroline,  When  I  want  to  see  an  object  above  me,  I  look 
up  ;  when  an  object  below  me,  I  look  down.  Does  not  this 
prove  that  I  see  the  objects  themselves  ?  for  if  I  beheld  only 
the  image,  there  would  be  no  necessity  for  looking  up  or  down^ 
according  as  the  object  was  higher  or  lower  than  myself. 


ON  OPTICS.  177 

Mrs.  B.  I  beg  your  pardon.  When  you  look  up  to  an 
elevated  object,  it  is  in  order  that  the  rays  reflected  from  it 
should  fall  upon  the  retina  of  your  eyes  ;  but  the  very  cir- 
cumstance  of  directing  your  eyes  upwards  convinces  you  that 
the  object  is  elevated,  and  teaches  you  to  consider  as  upper- 
most the  image  it  forms  on  the  retina,  though  it  is,  in  fact^ 
represented  in  the  lowest  part  of  it.  When  yo,.  look  down 
upon  an  object,  you  draw  your  conclusion  from  a  similar 
reasoning  ;  it  is  thus  that  we  see  all  objects  in  the  direction 
of  the  rays  which  reach  our  eyes. 

But  I  have  a  further  proof  in  favor  of  what  I  have  advanced, 
which  I  hope  will  remove  your  remaining  doubts  ;  I  shall, 
however,  defer  it  till  our  next  meeting,  as  the  lesson  has  beea 
sufficiently  long  to-day. 


CONVERSATION  XV. 


OPTICS— CONTINUED. 

ON  THE  ANGLE  OF  VISION,  AND  THE    REFLECTION    OF 
MIRRORkS 

Angle  of  Vision  ;  Reflection  of  Plain  Mirrors  ;  Reflection 
of  Convex  Mirrors  ;  Reflection  of  Concave  Mirrors. 


CAROLINE. 

Well,  Mrs.  B.,  I  am  very  impatient  to  hear  what  further 
proofs  you  have  to  offer  in  support  of  your  theory.  You  must 
allow  that  it  was  rather  provoking  to  dismiss  us  as  you  did  at 
our  last  meeting. 

Mrs.  B,  You  press  so  hard  upon  me  with  your  objections, 
that  you  must  give  me  time  to  recruit  my  forces. 

Can  you  tell  me,  Caroline,  why  objects  at  a  distance  appear 
smaller  than  they  really  are  ? 

Caroline,     I  know  no  other  reason  than  their  distance. 

Mrs.  B.  I  do  not  think  I  have  more  cause  to  be  satisfied 
with  your  reasons,  than  you  appear  to  be  with  mine. 

We  must  refer  again  to  the  camera  obscura  to  account  for 
this  circumstance  and  you  will  find,  that  the  different  apparent 
dimensions  of  objects  at  different  distances,  proceed  from  our 
seeing,  not  the  objects  themselves,  but  merely  their  image  on 
the  retina.  Fig.  1 .  plate  XVII.  represents  a  row  of  trees,  as 
viewed  in  the  camera  obscura.  I  have  expressed  the  direction 
of  the  rays,  from  the  objects  to  the  image,  by  lines.  Now, 
observe,  the  ray  which  comes  from  the  top  of  the  nearest  tree, 
and  that  which  comes  from  the  foot  of  the  same  tree,  meet  at 
the  aperture,  forming  an  angle  of  about  twenty-five  degrees  ; 
this  is  called  the  angle  of  vision,  under  which  we  see  the  tree. 


""■■■^ 


M 


ON  TH5E  ANGLE  OP  VISION.  l79 

These  Yays  cross  each  other  at  the  aperture,  forming  equal 
angles  on  each  side  of  it,  and  represent  the  tree  inverted  in  the 
camera  obscura.  The  degrees  of  the  image  are  considerably 
smaller  than  those  of  the  object^  but  the  proportions  are 
perfectly  preserved. 

Now  let  us  notice  the  upper  and  lower  ray,  from  the  most 
distant  tree  ;  they  form  an  angle  of  not  more  than  twelve  or 
fifteen  degrees,  and  an  image  of  proportional  dimensions. 
Thus,  two  objects  of  the  same  size,  as  the  two  trees  of  the 
avenue,  form  figures  of  different  sizes  in  the  camera  obscura, 
according  to  their  distance  ;  or  in  other  words,  according  to 
the  angle  of  vision  under  which  they  are  seen.  Do  you 
understand  this  ? 

Caroline,     Perfectly. 

Mrs.  B.  Then  you  have  only  to  suppose  that  the  repre- 
sentation in  the  camera  obscura  is  similar  to  that  on  the  retina. 

Now  since  objects  in  the  same  magnitudes  appear  to  be  of 
different  dimensions,  when  at  different  distances  from  us,  let 
me  ask  you,  which  it  is  that  we  see  ;  the  real  objects,  which 
we  know  do  not  vary  in  size,  or  the  images,  which  we  know 
do  vary  according  to  the  angle  of  vision  under  which  we  see 
them  ? 

Caroline,  I  must  confess,  that  reason  is  in  favor  of  the 
latter.  But  does  that  chair  at  the  further  end  of  the  room 
form  an  image  on  my  retina  much  smaller  than  this  which  is 
close  to  me  ?  they  appear  exactly  of  the  same  size. 

Mrs,  B.  I  assure  you  they  do  not.  The  experience  we 
acquire  by  the  sense  of  touch  corrects  the  errors  of  our  sight 
with  regard  to  objects  within  our  reach.  You  are  so  perfectly 
convinced  of  the  real  size  of  objects  which  you  can  handle, 
that  you  do  not  attend  to  their  apparent  difference. 

Does  that  house  appear  to  you  much  smaller  than  when  you 
are  close  to  it  ? 

Caroline,     No,  because  it  is  very  near  us. 

Blrs.  B.  And  yet  you  can  see  the  whole  of  it  through  one 
of  the  windows  of  this  room.  The  image  of  the  house,  on 
your  retina,  must,  therefore,  be  smaller  than  that  of  the  win- 
dow through  which  you  see  it.  It  is  your  knowledge  of  the 
real  size  of  the  house  which  prevents  your  attending  to  its 
apparent  magnitude.  If  you  were  accustomed  to  draw  from 
nature,  you  would  be  fully  aware  of  this  difference. 

Emily,  And  pray,  what  is  the  reason  that,  when  we  look 
'10  an  avenue,  the  trees  not  only  appear  smaller  as  they  are 


ISO  ON  THE  ANGLE  OF  VISION. 

more  distant,  but  seem  gradually  to  approach  each  other  till 
they  meet  in  a  point  ? 

Mrs.  B.  Not  only  the  trees,  but  the  road  which  separates 
the  two  rows,  forms  a  smaller  visual  angle,  in  proportion  as  it 
is  more  distant  from  us  ;  therefore  the  width  of  the  road 
gradually  diminishes  as  well  as  the  size  of  the  trees,  till  at 
length  the  road  apparently  terminates  in  a  point,  at  which  the 
trees  seem  to  meet. 

But  this  eftect  of  the  angle  of  vision  will  be  more  fully 
illustrated  by  a  little  model  of  an  avenue,  which  I  have  made 
for  that  purpose.  It  consists  of  six  trees,  leading  to  a  hex- 
agonal temple,  and  viewed  by  an  eye,  on  the  retina  of  which 
the  picture  of  the  objects  is  delineated. 

I  beg  that  you  will  not  criticise  the  proportions  ;  for  though 
the  eye  is  represented  the  size  of  life,  while  the  trees  are  not 
jRore  than  three  inches  high,  the  disproportion  does  not  affect 
the  principle,  which  the  model  is  intended  to  elucidate. 

Emily.  The  threads  which  pass  from  the  objects  through 
the  pupil  of  the  eye  to  the  retina,  are,  I  suppose,  to  represent 
the  rays  of  light  which  convey  the  image  of  the  objects  to  the 
retina  ? 

Mrs.  B.  Yes.  I  have  been  obliged  to  limit  the  rays  to  a 
very  small  number,  in  order  to  avoid  confusion  ;  there  are, 
you  see,  only  two  from  each  tree. 

Caroline.  But  as  one  is  from  the  summit,  and  the  other 
from  the  foot  of  the  tree,  they  exemplify  the  different  angles 
under  which  we  see  objects  at  different  distances,  better  than 
if  there  were  more. 

Mrs.  B.  There  are  seven  rays  proceeding  from  the  temple, 
one  from  the  summit,  and  two  from  each  of  the  angles  that 
are  visible  to  the  eye,  as  it  is  situated ;  from  these  you  may 
form  a  ]ust  idea  of  the  difference  of  the  angle  of  vision  of 
objects  viewed  obliquely,  or  in  front ;  for  though  the  six  sides 
of  the  temple  are  of  equal  dimensions,  that  which  is  opposite 
to  the  eye  is  seen  under  a  much  larger  angle,  than  those  which 
are  viewed  obliquely.  It  is  on  this  principle  that  the  laws  of 
perspective  are  founded. 

Emily.  I  am  very  glad  to  know  that,  for  I  have  lately 
begun  to  learn  perspective,  which  appeared  to  me  a  very 
dry  study  ;  but  now  that  I  am  acquainted  with  the  prin- 
ciples on  which  it  is  founded,  I  shall  find  it  much  more 
interesting. 

Caroline.     In  drawing  a  view  from  nature,  then,  we  do  not 


(^K  THE  ANGLE  OF  VISION.  181 

copy  the  real  objects,  but  the  image  they  form  on  the  retma 
of  our  eyes  ? 

Mrs,  B,  Certainly.  In  sculpture,  we  copy  nature  as  she 
really  exists  ;  in  painting,  we  represent  her  as  she  appears  to 
us.  It  was  on  this  account  that  I  found  it  difficult  to  explain 
by  a  drawing  the  efi'ects  of  the  angle  of  vision,  and  was  under 
the  necessity  of  constructing  a  model  for  that  purpose. 

Emily.  I  hope  you  will  allow  us  to  keep  this  model  some 
time,  in  order-lo  study  it  more  completely,  for  a  great  deal 
may  be  learned  from  it ;  it  illustrates  the  nature  of  the  angle 
of  vision,  the  apparent  diminution  of  distant  objects,  and  the 
inversion  of  the  image  on  the  retina.  But  pray,  why  are  the 
threads  that  represent  the  rays  of  light,  colored,  the  same  as 
the  objects  from  which  they  proceed  ? 

Mrs,  B,  That  is  a  question  which  you  must  excuse  my 
answering  at  present,  but  I  promise  to  explain  it  to  you  in  due 
time. 

I  consent  very  willingly  to  your  keeping  the  model,  on 
condition  that  you  will  make  an  imitation  of  it,  on  tlie  same 
principle,  but  representing  different  objects. 

We  must  now  conclude  the  observations  that  remain  to  be 
made  on  the  angle  of  vision. 

If  an  object,  with  an  ordinary  degree  of  illumination,  does 
not  subtend  an  angle  of  more  than  two  seconds  of  a  degree, 
it  is  invisible.  There  are  consequently  two  cases  in  which 
objects  may  be  invisible,  either  if  they  are  too  small,  or  sa 
distant  as  to  form  an  angle  less  than  two  seconds  of  a  degree. 

In  like  manner,  if  the  velocity  of  a  body  does  not  exceed 
20  degrees  in  an  hour,  its  motion  is  imperceptible. 

Caroline,  A  very  rapid  motion  may  then  be  imperceptible, 
provided  the  distance  of  the  moving  body  is  sufficiently  great. 

Mrs,  B.  Undoubtedly  ;  for  tlie  greater  its  distance  the 
smaller  will  be  the  angle  under  which  its  motion  will  appear 
to  the  eye.  It  is  for  this  reason  that  the  motion  of  the 
celestial  bodies  is  invisible,  notwithstanding  their  immense 
velocity. 

Emily,  I  am  surprised  that  so  great  a  velocity  as  20 
degrees  an  hour  should  be  invisible. 

Mrs,  B,  The  real  velocity  depends  altogether  on  the 
-space  comprehended  in  each  degree  ;  and  this  space  depends 
on  the  distance  of  the  object,  and  the  obliquity  of  its  path. 
Observe,  likewise,  that  we  cannot  judge  of  the  velocity  of  a 
body  in  motion  unless  we  know  its  distance  ;  for  supposing 
16 


182  ON  THE  ANGLE  OF  VISION. 

two  men  to  set  off  at  the  same  moment  from  A  and  B,  (fig.  2.) 
to  walk  each  to  the  end  of  their  respective  lines  C  and  D  ;  if 
they  perform  their  walk  in  the  same  space  of  time,  they  must 
have  proceeded  at  a  very  different  rate,  and  yet  to  an  eye 
situated  at  E,  they  will  appear  to  have  moved  with  equal 
velocity  :  because  they  will  both  have  gone  through  an  equal 
number  of  degrees,  though  over  a  very  unequal  length  of 
ground.  Sight  is  an  extremely  useful  sense  no  doubt,  but  it 
cannot  always  be  relied  on,  it  deceives  us  both  in  regard  to  the 
size  and  the  distance  of  objects  ;  indeed  our  senses  would  be 
very  liable  to  lead  us  into  error,  if  experience  did  not  set  us 
right. 

Emily.  Between  the  two,  I  think  that  we  contrive  to 
acquire  a  tolerably  accurate  idea  of  objects. 

Mrs.  B,  At  least  sufficiently  so  for  the  general  purposes  of 
life.  To  convince  you  how  requisite  experience  is  to  correct 
the  errors  of  sight,  I  shall  relate  to  you  the  case  of  a  young 
man  who  was  blind  from  his  infancy,  and  who  recovered  his 
sight  at  the  age  of  fourteen,  by  the  operation  of  couching. 
At  first  he  had  no  idea  either  of  the  size  or  distance  of  objects, 
but  imagined  that  every  thing  he  saw  touched  his  eyes  ;  and 
it  was  not  till  after  having  repeatedly  felt  them,  and  walked 
from  one  object  to  another  that  he  acquired  an  idea  of  their 
respective  dimensions,  their  relative  situations,  and  their 
distances. 

Caroline.  The  idea  that  objects  touched  his  eyes,  is  how- 
ever not  so  absurd  as  it  at  first  appears  ;  for  if  we  consider 
that  v/e  see  only  the  image  of  objects,  this  image  actually 
touches  our  eyes, 

Mrs.  B.  That  is  doubtless  the  reason  of  the  opinion  he 
formed,  before  the  sense  of  touch  had  corrected  his  judgment. 

Caroline,  But  since  an  image  must  be  formed  on  the 
retina  of  each  of  our  eyes,  why  do  we  not  see  objects  double  ? 

Mrs.  B.  The  action  of  the  rays  on  the  optic  nerve  of 
each  eye  is  so  perfectly  similar,  that  they  produce  but  a  single 
sensation,  the  mind  therefore  receives  the  same  idea,  from  the 
retina  of  both  eyes,  and  conceives  the  object  to  be  single. 

Caroline.  This  is  difficult  to  comprehend,  and,  I  should 
think,  can  be  but  conjectural. 

Mrs.  B.  I  can  easily  convince  you  that  you  have  a  dis- 
tinct image  of  an  object  formed  on  the  retina  of  each  eye. 
l^ook  at  the  bell-rope,  and  tell  me  do  you  see  it  to  the  right  or 
the  left  of  the  pole  of  the  fire-skreen  .^ 


KEFLECTION  OF  MIRRORS.  183 

Caroline,     A  little  to  the  right  of  h. 

Mrs,  B,  Then  shut  your  right  eye,  and  you  will  see  it  to 
the  left  of  the  pole. 

Caroline,     That  is  true  indeed  ! 

Mrs,  B,  There  are  evidently  two  representations  of  the 
bell-rope  in  different  situations,  which  must  be  owing  to  an 
image  of  it  being  formed  on  both  eyes  ;  if  the  action  of  the 
rays  therefore  on  each  retina  were  not  so  perfectly  similar  as 
to  produce  but  one  sensation,  we  should  see  double,  and  we 
find  that  to  be  the  case  with  many  persons  who  are  afflicted 
with  a  disease  in  one  eye,  which  prevents  the  rays  of  light 
from  affecting  it,  in  the  same  manner  as  the  other. 

Emily.  Pray,  jMrs.  B.,  when  we  see  the  image  of  an  object 
in  a  looking-glass,  why  is  it  not  inverted  as  in  the  camera 
obscura,  and  on  the  retina  of  the  eye  ? 

Mrs,  B,  Because  the  rays  do  not  enter  the  mirror  by  a 
small  aperture,  and  cross  each  other,  as  they  do  at  the  orifice 
of  a  camera  obscura,  or  the  pupil  of  the  eye. 

When  you  view  yourself  in  a  mirror,  the  rays  from  your 
eyes  fall  perpendicularly  upon  it,  and  are  reflected  in  the  same 
line  ;  the  image  is  therefore  described  behind  the  glass,  and  is 
situated  in  the  same  manner  as  the  object  before  it. 

Emily,  Yes,  I  see  that  it  is  ;  but  the  looking-glass  is  not 
nearly  so  tall  as  I  am,  how  is  it  therefore  that  I  can  see  the 
whole  of  my  figure  in  it  ? 

Mrs,  B,  It  is  not  necessary  that  the  mirror  should  be 
more  than  half  your  height,  in  order  that  you  may  see  the 
whole  of  your  person  in  it,  {fig,  3.)  The  ray  of  light  C  D 
from  your  eye,  which  falls  perpendicularly  on  the  mirror  B  D, 
will  be  reflected  back  in  the  same  line  ;  but  the  ray  from  your 
feet  will  fall  obliquely  on  the  mirror,  for  it  must  ascend  in 
order  to  reach  it  ;  it  will  therefore  be  reflected  in  the  line 
D  A  :  and  since  we  view  objects  in  the  direction  of  the  re- 
flected rays,  which  reach  the  eye,  and  that  the  image  appears 
at  the  same  distance  behind  the  mirror  that  the  object  is  be- 
fore it,  we  must  continue  the  line  A  D  to  E,  and  the  line  C  D 
to  F,at  the  termination  of  which,  the  image  will  be  represented. 

Emihj.  Then  I  do  not  understand  vv^hy  I  should  not  see 
the  whole  of  my  person  in  a  much  smaller  mirror,  for  a  ray 
of  light  from  my  feet  would  always  reach  it,  though  more 
obliquely. 

Mrs.  B.  True  ;  but  the  more  obliquely  the  ray  falls  on 
the  mirror,  the  more  obliquely  it  will  be  reflected  ,  the  ray 


184  REFLECTION  OF  MIRKOlliJ. 

would  therefore  be  reflected  above  your  head,  and  you  could 
not  see  it.     This  is  shown  by  the  dotted  line,     {fig,  3.) 

Now  stand  a  little  to  the  right  of  the  mirror,  so  that  the 
rays  of  light  from  your  figure  may  fell  obliquely  on  it — 

Emily.     There  is  no  image  formed  of  me  in  the  glass  now. 

Mrs,  B,  I  beg  your  pardon,  there  is  ;  but  you  cannot  see 
it,  because  the  incident  rays  falling  obliquely  on  the  mirror 
will  be  reflected  obliquely  in  the  opposite  direction,  the  angles 
of  incidence  and  of  reflection  being  equal.  Caroline,  place 
yourself  in  the  direction  of  the  reflected  rays,  and  tell  me 
whether  you  do  not  see  Emily's  image  in  the  glass  ? 

Caroline,  Let  me  consider.  In  order  to  look  in  the  di- 
rection of  the  reflected  rays,  I  must  place  myself  as  much  to 
the  left  of  the  glass  as  Emily  stands  to  the  right  of  it.  Now 
I  see  her  image  but  it  is  not  straight  before  me,  but  before 
her  ;  and  appears  at  the  same  distance  behind  the  glass,  as 
she  is  in  front  of  it. 

Mrs,  B,  You  must  recollect,  that  we  always  see  objects 
in  the  direction  of  the  last  rays  which  reach  our  eyes.  Figure 
4  represents  an  eye  looking  at  the  image  of  a  vase  reflected 
by  a  mirror  ;  it  must  see  it  in  the  direction  of  the  ray  A  B,  as 
that  is  the  ray  which  brings  the  image  to  the  eye  :  prolong 
the  ray  to  C,  and  in  that  spot  will  the  image  appear. 

Caroline,  I  do  not  understand  why  a  looking-glass  reflects 
the  rays  of  light :  for  glass  is  a  transparent  body  y/hich  should 
transmit  them  ?  -^^ 

Mrs,  B.  It  is  not  the  glass  that  reflects  the  rays  which 
form  the  image  you  behold,  but  the  mercury  behiiid  it.  The 
glass  acts  chiefly  as  a  transparent  case,  through  which  the 
rays  find  an  easy  passage. 

Caroline,  Why  then  should  not  mirrors  be  made  simply 
of  mercury  ? 

Mrs.  B,  Because  mercury  is  a  fluid.  By  amalgamating 
it^^kh  tin-foil,  it  becomes  of  the  consistence  of  paste,  attaches 
its^  to  the  glass,  and  forms  in  fact  a  mercurial  mirror,  which 
would  be  much  more  perfect  without  its  rrlass  cover,  for  the 
purest  glass  is  never  perfectly  transparent  ;  some  of  the  rays 
therefore  are  lost  during  their  passage  through  it,  by  being 
either  absorbed,  or  irregularly  reflected. 

This  imperfection  of  glass  mirrors  has  introduced  the  use 
of  metallic  mirrors,  for  opiical  purposes. 

Emily,  But  since  all  opaque  bodies  reflect  the  rays  of 
light,  I  do  not  understand  why  they  are  not  all  mirrors  ? 


REFLECTION  OF  CONVEX  MIRRORS.  185 

Caroline,  A  curious  idea  indeed,  sister  ;  it  would  be 
very  gratifying  to  see  one's  self  in  every  object  at  which  one 
looked. 

Mrs,  B,  It  is  very  true  that  all  opaque  objects  reflect 
light ;  but  the  surface  of  bodies  in  general  is  so  rough  and 
uneven,  that  their  reflection  is  extremely  irregular,  which 
prevents  the  rays  from  forming  an  image  on  the  retina.  This 
you  will  be  able  to  understand  better,  when  I  shall  explain 
to  3^ou  the  nature  of  vision,  and  the  structure  of  the  eye. 

You  may  easily  conceive  the  variety  of  directions  in  which 
rays  would  be  reflected  by  a  nutmeg-grater,  on  account  of  the 
inequality  of  its  surface,  and  the  number  of  holes  with  which 
it  is  pierced.  All  solid  bodies  resemble  the  nutmeg-grater  in 
these  respects,  more  or  less  ;  and  it  is  only  those  which  are 
susceptible  of  receiving  a  polish,  that  can  be  made  to  reflect 
the  rays  with  regularity.  As  hard  bodies  are  of  the  closest 
texture,  the  least  porous,  and  capable  of  taking  the  highest 
polish,  they  make  the  best  mirrors ;  none  therefore  are  so 
well  calculated  for  this  purpose  as  metals. 

Caroline,  But  the  property  of  regular  reflection  is  not 
confined  to  this  class  of  bodies ;  for  I  have  often  seen  myself 
in  a  highly  polished  mahogany  table. 

Mrs.  B,  Certainly  ;  but  as  that  substance  is  less  durable^ 
and  its  reflection  less  perfect,  than  that  of  metals,  I  believe  it 
would  seldom  be  chosen  for  the  purpose  of  a  mirror. 

There  are  three  kinds  of  mirrors  used  in  optics  ;  the  plain 
or  flat,  which  are  the  common  mirrors  we  have  just  mention- 
ed ;  convex  mirrors  ;  and  concave  mirrors.  The  reflection 
of  the  two  latter  is  very  difterent  from  that  of  the  former. 
The  plain  mirror,  we  have  seen,  does  not  alter  the  direction 
of  the  reflected  rays,  and  forms  an  image  behind  the  glass 
exactly  similar  to  the  object  before  it.  A  convex  mirror  has 
the  peculiar  property  of  making  the  reflected  rays  diverge^ 
by  which  means  it  diminishes  the  image  ;  and  a  concave 
mirror  makes  the  rays  converge,  and,  under  certain  circum- 
stances, magnifies  the  image. 

Emily,  We  have  a  convex  mirror  in  the  drawing-room,, 
which  forms  a  beautiful  miniature  picture  of  the  objects  in 
the  room  ;  and  1  have  often  amused  myself  with  looking  at 
my  magnified  face  in  a  concave  mirror.  But  I  hope  you  will 
explain  to  us  why  the  one  enlarges  while  the  other  diminishes 
the  objects  it  reflects. 

Mrs,  B,  Let  us  begin  by  examining  the  reflection  of  a 
16^ 


tbO  KEI  LECTION  OF  CONVEX  MIEKORS. 

convex  mirror.  This  is  formed  of  a  portion  of  the  exterior 
surface  of  a  sphere.  When  several  parallel  rays  fall  upon  it, 
that  ray  only,  which,  if  prolonged,  would  pass  through  the 
centre  or  axis  of  the  mirror,  is  pei*pendicular  to  it.  In  order 
to  avoid  confusion,  I  have,  in  fig.  1 .  plate  XVIII.  drawn  only 
three  parallel  lines,  A  B,  C  D,  E  F,  to  represent  rays  falling 
on  the  convex  mirror  M  N  ;  the  middle  ray,  you  will  ob- 
serve^ is  perpendicular  to  the  mirror,  the  others  fall  on  it 
obliquely. 

Caroline,  As  the  three  rays  are  parallel,  why  are  they  not 
all  perpendicular  to  the  mirror  ? 

Mrs,  B,  They  would  be  so  to  a  flat  mirror  ;  but  as  this  is 
sphericau  no  ray  can  fall  perpendicularly  upon  it  which  is  not 
directed  towards  the  centre  of  the  sphere. 

Emily,  Just  as  a  weight  falls  perpendicularly  to  the  earth 
when  gravity  attracts  it  towards  the  centre. 

Mrs,  B,  In  order,  therefore,  that  rays  may  fall  perpen- 
dicularly to  the  mirror  at  B  and  F,  the  rays  must  be  in  the 
direction  of  the  dotted  lines,  which,  you  may  observe,  meet 
at  the  centre  O  of  the  sphere,  of  which  the  mirror  forms  a 
portion. 

Now  can  you  tell  me  in  what  direction  the  three  rays* 
A  B,  C  D,  E  F,  will  be  reflected  ? 

Emily,  Yes,  I  think  so  :  the  middle  ray  falling  perpen- 
dicularly on  the  mirror,  will  be  reflected  in  the  same  line  :  the 
two  others  falling  obliquely,  will  be  reflected  obliquely  to 
G  H  ;  for  the  dotted  lines  you  have  drawn  are  perpendiculars- 
which  divide  their  angles  of  incidence  and  reflection. 

Mrs,  B,  Extremely  well,  Emily :  and  since  we  see  ob- 
jects in  the  direction  of  the  reflected  ray,  we  shall  see  the 
image  at  L,  which  is  the  point  at  which  the  reflected  rays,  if 
continued  through  the  mirror,  would  unite  and  form  an  image. 
This  point  is  equally  distant  from  the  surface  and  centre  of 
the  sphere,  and  is  called  the  imaginary  focus  of  the  mirror. 

Caroline,     Pray,  what  is  the  meaning  of  focus  ? 

Mrs,  B,  A  point  at  which  converging  rays  unite.  And 
it  is  in  this  case  called  an  imaginary  focus  ;  because  the  rays 
do  not  really  unite  at  that  point,  but  only  appear  to  do  so :  for 
f  he  rays  do  not  pass  through  the  mirror,  since  they  are  reflected 
by  it. 

Emily.  I  do  not  yet  understand  why  an  object  appears 
^mailer  when  viewed  in  a  convex  mirror. 

^frp,  /?.     It  is  owing  to  the  divergence  of  the  reflected 


PLATE.  XWL 


REFLECTION  OF  CONCAVE  MIRUORS.  ISf 

rays.  You  have  seen  that  a  convex  mirror  converts,  by 
reflection,  parallel  rays  into  divergent  rays  ;  rays  that  fail 
upon  the  mirror  divergent,  are  rendered  still  more  so  by 
reflection,  and  convergent  rays  are  reflected  either  parallel, 
or  less  convergent.  If  then  an  object  be  placed  before  any 
part  of  a  convex  mirror,  as  the  vase  A  B,  flg.  2.  for  instance, 
the  two  rays  from  its  extremities,  falling  convergent  on  the 
mirror,  will  be  reflected  less  convergent,  and  will  not  come  to 
a  focus  till  they  arrive  at  C  ;  then  an  eye  placed  in  the  direc- 
tion of  the  reflected  rays,  will  see  the  image  formed  in  (or 
rather  behind)  the  mirror  at  a  h, 

Caroline,  But  the  reflected  rays  do  not  appear  to  me  to 
converge  less  than  the  incident  rays.  1  should  have  supposed 
that,  on  the  contrary,  they  converged  more,  since  they  meet 
in  a  point  ? 

Mrs.  B.  They  would  unite  sooner  than  they  actually  do^ 
if  they  were  not  less  convergent  than  the  incident  rays  :  for 
observe,  that  if  the  incident  rays,  instead  of  being  reflected  by 
the  mirror,  continued  their  course  in  their  original  direction^ 
they  would  come  to  a  focus  at  D,  which  is  considerably  nearer 
to  the  mirror  than  at  C  ;  the  image  is  therefore  seen  under  a 
smaller  angle  than  the  object  ;  and  the  more  distant  the  latter 
is  from  the  mirror,  the  less  is  the  image  reflected  by  it. 

You  will  now  easily  understand  the  nature  of  the  reflection 
of  concave  mirrors.  These  are  formed  of  a  portion  of  the 
internal  surface  of  a  hollow  sphere,  and  their  peculiar  proper- 
ty is  to  converge  the  rays  of  light. 

Can  you  discover,  Caroline,  in  what  direction  the  three 
parallel  rays,  A  B,  C  D,  E  F,  which  fall  on  the  concave  mir- 
ror M  N  (fig.  3.)  are  reflected  ? 

Caroline,  I  believe  I  can.  The  middle  ray  is  sent  back 
in  the  same  line,  as  it  is  in  the  direction  of  the  axis  of  the 
mirror  ;  and  the  two  others  will  be  reflected  obliquely,  as  they 
fall  obliquely  on  the  mirror.  I  must  now  draw  two  dotted 
lines  perpendicular  to  their  points  of  incidence,  which  will 
divide  their  angles  of  incidence  and  reflection  ;  and  in  order 
that  those  angles  may  be  equal,  the  two  oblique  rays  must  be 
reflected  to  L,  where  they  will  unite  with  the  middle  ray. 

Mi's.  B.  Very  well  explained.  Thus  you  see  that  when 
any  number  of  parallel  rays  fall  on  a  concave  mirror  they  are 
all  reflected  to  a  focus  ;  for  in  proportion  as  the  rays  are 
more  distant  from  the  axis  of  the  mirror,  they  fall  more  ob- 
liquely upon  it,  and  are  more  obliquely  reflected  ;  in  conse- 


188  REFLECTION  OF  CONCAVE  MIRRORS. 

quence  of  which  they  come  to  a  focus  in  the  direction  of  the 
axis  of  the  mirror,  at  a  point  equally  distant  from  the  centre 
and  the  surface  of  the  sphere,  and  this  point  is  not  an  imagina- 
ry focus,  as  happens  with  the  convex  mirror,  but  is  the  true 
focus  at  which  the  rays  unite. 

Emihj,  Can  a  mirror  form  more  than  one  focus  by  reflect- 
ing rays? 

Mrs,  B,  Yes.  If  rays  fall  convergent  on  a  concave  mir- 
ror, (fig.  4.)  they  are  sooner  brought  to  a  focus,  L,  than  par- 
allel rays ;  their  focus  is  therefore  nearer  to  the  mirror  M 
N.  Divergent  rays  are  brought  to  a  more  distant  focus  than 
parallel  rays,  as  in  fig.  5.  where  the  focus  is  at  L  ;  but  the 
true  focus  of  mirrors,  either  convex  or  concave,  is  that  of 
parallel  rays,  which  is  equally  distant  from  the  centre,  and 
the  surface  of  the  sphere. 

I  shall  nov/  show  you  the  reflection  of  real  rays  of  light, 
by  a  metallic  concave  mirror.  This  is  one  made  of  polished 
tin,  which  I  expose  to  the  sun,  and  as  it  shines  bright,  we 
shall  be  able  to  collect  the  rays  into  a  very  brilliant  focus. 
I  hold  a  piece  of  paper  where  I  imagine  the  focus  to  be  situ- 
ated ;  3  ou  may  see  by  the  vivid  spot  of  light  on  the  paper, 
how  much  the  rays  converge  :  but  it  is  not  yet  exactly  in  the 
focus  ;  as  I  approach  the  paper  to  that  point,  observe  hov/ 
the  brightness  of  the  spot  of  light  increases^  while  its  size 
diminishes. 

Caroline,  That  must  be  occasioned  by  the  rays  becoming 
closer  together.  I  think  you  hold  the  paper  just  in  the  focus 
now,  the  light  is  so  small  and  dazzling — Oh,  Mrs.  B.,  the 
paper  has  taken  fire  ! 

Mrs,  B,  The  rays  of  light  cannot  be  concentrated,  with- 
out, at  the  same  time,  accumulating  a  proportional  quantity 
of  heat  :  hence  concave  mirrors  have  obtained  the  name  of 
burning-mirrors. 

Ennly,  I  have  often  heard  of  the  surprising  effects  of 
burning-mirrors,  and  I  am  quite  delighted  to  understand  their 
nature. 

Caroline,  It  cannot  be  the  true  focus  of  the  mirror  at 
which  the  rays  of  the  sun  unite,  for  as  they  proceed  from  a 
point,  they  must  fall  divergent  upon  the  mirror. 

Mrs,  B,  Strictly  speaking,  they  certainly  do.  But  when 
rays  come  from  such  an  immense  distance  as  the  sun,  their 
divergence  is  so  trifling,  as  to  be  imperceptible  ;  and  they 
may  be  considered  as  parallel :  their  point  of  union  is,  there- 


THE  REFLECTION  OF  MIRRORS.  189 

fore,  the  true  focus  of  the  mirror^  and  there  the  image  of  the 
object  is  represented. 

Now  that  I  have  removed  the  mirror  out  of  the  influence  of 
the  sun's  rays,  if  I  place  a  burning  taper  in  the  focus,  how 
will  its  light  be  reflected  ?  (iig.  6.) 

Caroline*     That,  I  confess,  I  cannot  say. 

Mrs.  B,  The  ray  which  falls  in  the  direction  of  the  axis 
of  the  mirror,  is  reflected  back  in  the  same  line  ;  but  let  us 
drav>^  tv>^o  other  rays  from  the  focus,  falling  on  the  mirror  at 
B  and  F  ;  the  dotted  linob  are  perpendicular  to  those  points, 
and  die  two  rays  will  thereibre  be  reflected  to  A  and  E. 

Caroline,  Oh,  now  I  understand  it  clearly.  The  rays 
which  proceed  from  a  light  placed  in  the  focus  of  a  concave 
mirror  fall  divergent  upon  it,  and  are  reflected  parallel.  It  is 
exactly  the  reverse  of  the  former  experiment,  in  which  the  sun's 
rays  fell  parallel  on  the  mirror,  and  were  reflected  to  a  focus. 

Mrs.  B,  Yes  :  when  the  incident  rays  are  parallel,  the 
reflected  rays,  converge  to  a  focus  ;  when,  on  the  contrary, 
the  incident  rays  proceed  from  the  focus,  they  are  reflected 
parallel.  This  is  an  important  law  of  optics,  and  since  you 
are  now  acquainted  with  the  principles  on  which  it  is  founded, 
I  hope  that  you  will  not  forget  it. 

Caroline,  I  am  sure  that  we  shall  not.  But,  Mrs.  B., 
you  said  that  the  image  was  formed  in  the  focus  of  a  concave 
mirror  ;  yet  I  have  frequently  seen  glass  concave  mirrors, 
where  the  object  has  been  represented  within  the  mirror,  in 
the  same  manner  as  in  a  convex  mirror. 

Mrs,  B,  That  is  the  case  only,  when  the  object  is  placed 
between  the  mirror  and  its  focus  ;  the  image  then  appears 
magnified  behind,  or,  as  you  call  it,  within  the  mirror. 

Caroline,  I  do  not  understand  why  the  image  should  be 
larger  than  the  object, 

Mrs.  B,  It  proceeds  from  the  convergent  property  of  the 
concave  mirror.  If  an  object,  A  B,  (fig.  7.)  be  placed  between 
the  mirror  and  its  focus,  the  rays  from  its  extremities  fall 
divergent  on  the  mirror,  and  on  being  reflected,  becomes  less 
divergent,  as  if  they  proceeded  from  C  :  to  an  eye  placed  in 
that  situation  the  image  will  appear  magnified  behind  the  mirror 
at  a  6,  since  it  is  seen  under  a  larger  angle  than  the  object. 

You  now,  I  hope,  understand  the  reflection  of  light  by 
opaque  bodies.  At  our  next  meeting,  we  shall  enter  upon 
another  property  of  light  no  less  interesting  which  is  called 
refraction. 


CONVERSATION  XVI. 


ONREFRACTIOxN  AND  COLORS. 

Transmission  of  Light  by  Transparent  Bodies  ;  Refrac- 
tion ;  Refraction  of  the  Atmosphere  ;  Refraction  of  a 
Lens  ;  Refraction  of  the  Frism  ;  Of  the  Colors  of  Rays 
of  Light  ;   Of  the  Colors  of  Bodies, 


MRS.  B. 

The  refraction  of  light  will  furnish  the  subject  of  to-day's 
tesson. 

Caroline,  That  is  a  property  of  which  I  have  not  the 
iaintest  idea. 

Mrs.  jB.  It  is  the  effect  which  transparent  mediums  pro- 
duce on  light  in  its  passage  through  them.  Opaque  bodies, 
you  know,  reflect  the  rays,  and  transparent  bodies  transmit 
them  ;  but  it  is  found,  that  if  a  ray,  in  passing  from  one 
medium  into  another  of  different  density,  fall  obliquely,  it  is 
turned  out  of  its  course. 

Caroline*  It  must  then  be  acted  on  by  some  new  power, 
otherwise  it  would  not  deviate  from  its  first  direction. 

Mrs.  B,  The  power  which  causes  the  deviation  of  the 
ray  appears  to  be  the  attraction  of  the  denser  medium.  Let 
us  suppose  the  two  mediums  to  be  air  and  water  ;  if  a  ray  of 
light  passes  from  air  into  water  it  is  more  strongly  attracted 
by  the  latter  on  account  of  its  superior  density. 

Emily.     In  what  direction  does  the  water  attract  the  ray  ? 

Mrs.  B.  It  must  attract  it  perpendicularly  towards  it,  in 
the  same  manner  as  gravity  acts  on  bodies. 

If  then  a  ray  A  B,  (fig.  1.  plate  XIX.)  fall  perpendicularly 
on  water,  the  attraction  of  the  water  acts  in  the  same  direction 
as  the  course  of  the  ray :  it  will  not  therefore  cause  a  devia« 


Fiff.  1. 


A''-   ^  TLATE.Xn: 


E  ]f  D 


Fiq.   4. 


T«E  REFRACTION  OF  LIGHT.  191 

tioiij  and  the  ray  will  proceed  straight  on  to  E.  But  if  it  fall 
obliquely,  as  the  ray  C  B,  the  water  will  attract  it  out  of  its 
course.  Let  us  suppose  the  ray  to  have  approached  the  sur- 
face of  a  denser  medium,  and  that  it  there  begins  to  be  af- 
fected by  its  attraction  ;  this  attraction,  if  not  counteracted 
by  some  other  power,  would  draw  it  perpendicularly  to  the 
water,  at  B  ;  but  it  is  also  impelled  by  its  projectile  force, 
which  the  attraction  of  the  denser  medium  cannot  overcome  ; 
the  ray,  therefore,  acted  on  by  both  these  powers,  moves  in  a 
direction  between  them,  and  instead  of  pursuing  its  original 
course  to  D,  or  being  implicitly  guided  by  the  water  to  E, 
proceeds  towards  F,  so  that  the  ray  appears  bent  or  broken. 

Caroline,  I  understand  that  very  well  ;  and  is  not  this 
the  reason  that  oars  appear  bent  in  water  ? 

Mrs.  B,  It  is  owing  to  the  refraction  of  the  rays  reflected 
by  the  oar  ;  but  this  is  in  passing  from  a  dense  to  a  rare  me- 
dium, for  you  know  that  the  rays,  by  means  of  which  you  see 
the  oar,  pass  from  water  into  air, 

Emily.  But  I  do  not  understand  why  a  refraction  takes 
place  when  a  ray  passes  from  a  dense  into  a  rare  medium  ;  I 
should  suppose  that  it  would  be  rather  less,  than  more 
attracted  by  the  latter. 

Mrs.  B.  And  it  is  precisely  on  that  account  that  the  ray 
is  refracted.  C  B,  fig.  2.  represents  a  ray  passing  obliquely 
from  glass  into  water  :  glass  being  the  denser  medium,  the 
ray  will  be  more  strongly  attracted  by  that  which  it  leaves 
than  by  that  which  it  enters.  The  attraction  of  the  glass 
acts  in  the  direction  A  B,  while  the  impulse  of  projection 
would  carry  the  ray  to  F  ;  it  moves  therefore  between  these 
directions  towards  D. 

Emily.  So  that  a  contrary  refraction  takes  place  when  a 
ray  passes  from  a  dense  into  a  rare  medium. 

Caroline.  But  does  not  the  attraction  of  the  denser  medium 
affect  the  ray  before  it  touches  it  ? 

Mrs.  B.  The  distance  at  which  the  attraction  of  the  den- 
ser medium  acts  upon  a  ray  is  so  small  as  to  be  insensible  ;  it 
appears  therefore  to  be  refracted  only  at  the  point  at  which  it 
passes  from  one  medium  to  the  other. 

Now  that  you  understand  the  principle  of  refraction,  I  will 
show  you  Jhe  refraction  of  a  real  ray  of  light.  Do  you  see 
the  flower  painted  at  the  bottom  of  the  inside  of  this  tea-cup  ? 
(Fig.  3.) 


102  THE  REFRACTION  OP  LIGHT. 

Emily.  Yes.  But  now  you  have  moved  it  just  out  of 
sight,  the  rim  of  the  cup  hides  it. 

Mrs.  B.  Do  not  stir.  I  will  fill  the  cup  with  water,  and 
you  will  see  the  flower  again. 

Emily.  I  do  indeed  !  Let  me  try  to  explain  this  :  when 
you  drew  the  cup  from  me  so  as  to  conceal  the  flower,  the 
rays  reflected  by  it  no  longer  met  my  eyes,  but  were  directed 
above  them  ;  but  now  that  you  have  filled  the  cup  with  water, 
they  are  refracted  by  the  attraction  of  the  water,  and  bent 
downwards  so  as  again  to  enter  my  eyes. 

Mrs.  B.  You  have  explained  it  perfectly :  Fig.  3.  wiir 
help  to  imprint  it  on  your  memory.  You  must  observe  that 
when  the  flower  becomes  visible  by  the  refraction  of  the  ray, 
3^ou  do  not  see  it  in  the  situation  which  it  really  occupies,  but 
an  image  of  the  flower  higher  in  the  cup ;  for  as  objects  always 
appear  to  be  situated  in  the  direction  of  the  rays  which  enter 
the  eye,  the  flower  will  be  seen  in  the  direction  of  the  reflected 
ray  at  B. 

Emily,  Then,  when  we  see  the  bottom  of  a  clear  stream 
of  water,  the  rays  which  it  reflects  being  refracted  in  their 
passage  from  the  water  into  the  air,  will  make  the  bottom 
appear  higher  than  it  really  is. 

Mrs.  B.  And  the  water  will  consequently  appear  more 
shallow.  Accidents  have  frequently  been  occasioned  by  this 
circumstance  ;  and  boys  who  are  in  the  habit  of  bathing 
should  be  cautioned  not  to  trust  to  the  apparent  shallowness 
of  water,  as  it  will  always  prove  deeper  than  it  appears  ; 
unless,  indeed,  they  view  it  from  a  boat  on  the  water,  which 
will  enable  them  to  look  perpendicularly  upon  it  ;  when  the 
rays  from  the  bottom  passing  perpendicularly,  no  refraction 
will  take  place. 

The  refraction  of  light  prevents  our  seeing  the  heavenly 
bodies  in  their  real  situation  ;  the  light  they  send  to  us  being 
refracted  in  passing  into  the  atmosphere,  we  see  the  sun  and 
stars  in  the  direction  of  the  refracted  ray  ;  as  described  in  fig. 
4.  plate  XIX.,  the  dotted  line  represents  the  extent  of  the 
atmosphere,  above  a  portion  of  the  earth,  E  B  E  :  a  ray  of 
light  coming  from  the  sun  S  falls  obliquely  on  it,  at  A,  and  is 
refracted  to  B  :  then,  since  we  see  the  object  in  the  direction 
of  the  refracted  ray,  a  spectator  at  B  will  see  an  image  of  the 
sun  at  C,  instead  of  the  real  object  at  S. 

Emily.     But  if  the  sun  were  immediately  over  our  heads, 


THE  REFRACTION  OP  LIGHT.  .  193 

it  s  rays  failing  perpendiculariy  on  tlie  atmosphere  would  not 
be  refractedj  and  we  sliould  tlien  see  the  real  sun,  in  its  true 
situation. 

Mi's,  B.  You  must  recollect  that  the  sun  is  vertical  only 
to  the  inhabitants  of  the  torrid  zone  ;  its  rays,  thereforCj  are 
always  refracted  in  these  climates.  There  is  also  anotliier 
obstacle  to  our  seeing  the  heavenly  bodies  in  their  real  situa- 
tions ;  light,  though  it  moves  with  extreme  velocity,  is  about 
eight  minutes  and  an  half  in  its  passage  from  the  sun  to  the 
earth  :  therefore,  when  the  rays  reach  us,  the  siui  must  have 
quitted  the  spot  he  occupied  on  their  departure  ;  yet  we  see 
him  in  the  direction  of  those  rays,  and  consequently  in  a 
situation  wliich  he  had  abandoned  eight  minutes  and  an  half 
before. 

Emily,  When  you  speak  of  the  sun's  motion,  you  mean, 
I  suppose,  his  apparent  motion,  produced  by  the  diurnal 
motion  of  the  earth  ? 

Mrs,  B.  No  doubt ;  the  eflect  being  the  same,  whether 
it  is  our  earth,  or  the  heavenly  bodies  which  move  :  it  is 
more  easy  to  represent  things  as  they  appear  to  be,  than  as 
the}^  realh"  are. 

Caroline.  During  the  morning,  then,  when  the  sun  is 
rising  towards  the  meridian,  v/e  must  (from  the  length  of  time 
the  light  is  in  reaching  us)  see  an  image  of  the  sun  below  that 
spot  which  it  really  occupies. 

Emily.  But  the  refraction  of  the  atmosphere  counteract- 
ing this  effect,  we  may  perhaps,  between  the  two,  see  the  sun 
in  its  real  situation. 

Caroline.  And  in  the  afternoon,  when  the  sun  is  sinking 
in  the  west,  refraction  and  the  length  of  time  which  the  light 
is  in  reaching  the  earth,  will  conspire  to  render  the  image  of 
the  sun  higher  than  it  really  is. 

Mrs.  B.  The  refraction  of  the  sun's  rays  by  the  atmos- 
phere prolongs  our  days,  as  it  occasions  our  seeing  an  image 
of  the  sun  both  before  he  rises  and  after  he  sets  ;  for  below 
the  horizon,  he  still  shines  upon  the  atmosphere,  and  his  rays 
are  thence  refracted  to  the  earth.  So  likewise  we  see  an 
image  of  the  sun  before  he  rises,  the  rays  that  previously  fall 
upon  the  atmosphere  being  reflected  to  the  earth. 

Caroline.  On  the  other  hand  we  must  recollect  that  light 
is  eight  minutes  and  an  half  on  its  journey  ;  so  that,  by  the 
time  it  reaches  the  earth,  the  sun  may  perhaps  be  risen  above 
the  horizon. 

17 


194  THE  REFRACTION  OF  LIGHT. 

Emily,     Pray,  do  not  glass  windows  refract  the  llglit  r 

Mrs,  B.  They  do  ;  but  this  refraction  is  not  perceptible, 
because,  in  passing  through  a  pane  of  glass  the  rays  suffer 
two  refractions,  wiiich  being  in  contrary  directions,  produce 
the  same  effect  as  if  no  refraction  had  taken  place. 

Emily,     I  do  not  understand  that. 

Mrs,  B,  Fig.  5.  plate  XIX.  will  make  it  clear  to  you  : 
A  A  represents  a  thick  pane  of  glass  seen  edgeways.  When 
the  ray  B  approaches  tlie  glass  at  C,  it  is  refracted  by  it ; 
and  instead  of  continuing  its  course  in  the  same  direction,  as 
the  dotted  line  describes,  it  passes  through  the  pane  to  D  ;  at 
that  point  returning  into  the  air,  it  is  again  refracted  by  the 
glass,  but  in  a  contrary  direction  to  the  first  refraction,  and 
in  consequence  proceeds  to  E.  Now  you  must  observe  that 
the  ray  B  C  and  the  ray  D  E  being  parallel,  the  light  does  not 
appear  to  have  suffered  any  refraction. 

Emily,  So  that  the  effect  v/hich  takes  place  on  the  ray 
entering  the  glass,  is  undone  on  its  quitting  it.  Or,  to  express 
myself  more  scientifically,  when  a  ray  of  light  passes  from 
one  medium  into  another,  and  through  that  into  the  first  again, 
the  two  refractions  being  equal  and  in  opposite  directions,  no 
sensible  effect  is  produced. 

Mr«.  B,  This  is  the  case  when  the  two  surfaces  of  the 
refracting  medium  are  parallel  to  each  other  ;  if  they  are  not, 
the  two  refractions  may  be  made  in  the  same  direction,  as  I 
shall  show  you. 

When  parallel  rays  (fig.  6.)  fall  on  a  piece  of  glass  having 
a  double  convex  surface,  and  which  is  called  a  Lens^  that 
only  which  falls  in  the  direction  of  the  axis  of  the  lens  is 
perpendicular  to  the  surface  ;  the  other  rays  falling  obUquely 
are  refracted  towards  the  axis,  and  will  meet  at  a  point  be- 
yond the  lens  called  its  focus. 

Of  the  three  rays,  ABC,  which  fall  on  the  lens  D  E,  the 
rays  A  and  C  are  refracted  in  their  passage  through  it,  to  a, 
and  c,  and  on  quitting  the  lens  they  undergo  a  second  refrac- 
tion in  the  same  direction  which  unites  them  with  the  ray  B, 
at  the  focufej  F. 

Emily,  And  what  is  the  distance  of  the  focus  from  the 
surface  of  the  lens  ? 

Mrs,  B,  The  focal  distance  depends  both  upon  the  form 
of  the  lens,  and  of  the  refractive  power  of  the  substance  of 
which  it  is  made  :  in  a  glass  lens,  both  sides  of  which  are 
equally  convex,  the  focus  is  situated  nearly  at  the  centre  of 


PLATE  XL 


ON  KEFRAC TION  AND  COLORS.  195 

the  Sphere  of  which  the  surface  of  the  lens  forms  a  portion  ; 
it  is  at  the  distance^  therefore,  of  the  radius  of  the  sphere. 

There  are  lenses  of  various  forms,  as  you  will  find  describ- 
ed in  fig.  1.  plate  XX.  The  property  of  those  which  have  a 
convex  surface  is  to  collect  the  rays  of  light  to  a  focus  ;  and 
of  those  which  have  a  concave  surface,  on  the  contrary,  to 
disperse  them.  For  the  rays  A  C  falling  on  the  concave 
lens  X  Y,  (fig.  7.  plate  XIX.)  instead  of  converging  towards 
the  ray  B,  which  fails  on  the  axis  of  the  lens,  will  each  be 
attracted  tow^ards  the  thick  edges  of  the  lens,  both  on  entering 
and  quitting  it,  and  will,  therefore,  by  the  first  refraction,  be 
made  to  diverge  to  a,  r,  and  by  the  second  to  d,  e. 

Caroline,  And  lenses  which  have  one  side  flat  and  the 
otiier  convex  or  concave,  as  A  and  B,  fig.  1,  plate  XX.,  are, 
I  suppose,  less  powerful  in  their  refractions  ? 

Mrs,  B.  Yes  ;  they  are  called  plano-convex,  and  plano- 
concave lenses :  the  focus  of  the  former  is  at  the  distance  of 
the  diameter  of  a  sphere,  of  which  the  convex  surface  of  the 
lens  forms  a  portion  ;  as  represented  in  fig.  2.  plate  XX. 
The  three  parallel  rays,  ABC,  are  brought  to  a  focus  by  the 
plano-convex  lens,  X  Y  at  F. 

I  must  now  explain  to  you  the  refraction  of  a  triangular 
pieceof  glass,  called  a  prism.     (Fig.  3.) 

Emily,  The  three  sides  of  this  glass  are  flat ;  it  cannot 
therefore  bring  the  rays  to  a  focus  ;  nor  do  I  suppose  that  its 
refraction  will  be  similar  to  that  of  a  flat  pane  of  glass, 
because  it  has  not  two  sides  parallel  ;  I  cannot  there- 
fore conjecture  what  eflect  the  rehaction  of  a  prism  can 
produce. 

Mrs.  B,  The  refractions  of  the  light,  on  entering  and  on 
quitting  the  prism,  are  both  in  the  same  direction.  (Fig.  3.) 
On  entering  the  prism  F,  the  ray  A  is  refracted  from  B  to  C, 
and  on  quitting  it  from  C  to  D. 

I  will  show  you  this  in  nature  ;  but  for  this  purpose  it  will 
be  advisable  to  close  the  window-shutters,  and  admit,  through 
the  small  aperture,  a  ray  of  light,  which  I  shall  refract  by 
means  of  this  prism. 

Caroline,  Oh,  what  beautiful  colors  are  represented  on 
the  opposite  wall  !  There  are  all  the  colors  of  the  rainbow, 
and  with  a  brightness  I  never  saw  equalled.  (Fig.  4.  plate 
Xa.  j 

Emihj,  I  have  seen  an  effect,  in  some  respect  similar  to 
this,  produced  by  tjie  rays  of  the  sun  shining  upon  glass 


19G  ON  REFilACTION  AND  COLOKS, 

lustres  ;  but  how  is  it  possible  that  a  piece  of  wiiite  glass  cun 
produce  such  a  variety  of  brilliant  colors  ? 

Mrs.  ii.  The  colors  are  not  formed  by  the  prism,  but 
existed  in  the  ray  previous  to  its  refraction. 

Caroline,  Yet^  before  its  refraction,  it  appeared  perfectly 
wliite. 

Mrs,  B.  The  white  rays  of  the  sun  are  composed  of 
colored  rays,  which,  v/hen  blended  together,  appear  colorless 
or  white. 

Sir  Isaac  Newton.,  to  whom  we  are  indebted  for  the  most 
important  discoveries  respecting  light  and  colors,  was  the 
first  who  divided  a  white  ray  of  light,  and  found  it  to  consist 
of  an  assemblage  of  colored  rays,  which  formed  an  image 
upon  the  wall,  such  as  you  now  see  exhibited,  (fig.  4.)  in 
which  are  displa3'ed  the  following  series  of  colors  :  red, 
orange,  yellow,  green,  blue,  indigo,  and  violet. 

Erailij,     But  how  does  a  prism  separate  these  colored  rays  ? 

Mrs,  B.  By  refraction.  It  appears  that  the  colored  rays 
have  different  degrees  of  refrangibility  ;  in  passing  through 
the  prism,  therefore,  they  take  different  directions  according 
to  their  susceptibility  of  refraction.  The  violet  rays  deviate 
most  from  their  original  course ;  they  appear  at  one  of  the 
ends  of  the  spectrum  A  B  :  contiguous  to  the  violet,  are  the 
blue  rays,  being  those  which  have  somewhat  less  refrangi- 
bility ;  then  follow,  in  succession,  the  green,  yellow,  orange, 
and,  lastly,  the  red,  which  are  the  least  refrangible  of  the 
colored  rays. 

Caroline,  I  cannot  conceive  how  these  colors,  mixed 
together,  can  become  white  ? 

Mrs.  B,  That  I  cannot  pretend  to  explain  ;  but  it  is  a 
fact  that  the  union  of  these  colors,  in  the  proportions  in  which 
they  appear  in  the  spectrum,  produce  in  us  the  idea  of  white- 
ness. If  you  paint  a  card  in  compartments  with  these  sevei\ 
colors,  and  whirl  it  rapidly  on  a  pin,  it  will  appear  white. 

But  a  more  decisive  proof  of  the  composition  of  a  white  ray 
is  afforded  by  reuniting  these  colored  rays,  and  forming  with 
them  a  ray  of  white  light. 

Caroline,  If  you  can  take  a  ray  of  white  light  to  pieces, 
and  put  it  together  again,  I  shall  be  quite  satisfied. 

Mis,  B,  This  can  be  done  by  letting  the  colored  rays, 
which  have  been  separated  by  a  prism,  fall  upon  a  lens, 
which  will  converge  them  to  a  focus  ;  and  if,  when  thus 
reunited,  we  find  that  they  appear  white  as  they  did  before 


ON  REFHACTION  AND  COLOrvS.  197 

refractioDj  I  hope  that  you  will  be  convinced  that  the  white 
rays  are  a  compound  of  the  several  colored  rays.  The  prism 
P,  3^ou  see,  (iig.  5.)  separates  a  ray  of  white  light  into  seven 
colored  rays,  and  the  lens  L  L  brings  them  to  a  focus  at  F, 
where  they  again  appear  v/hite. 

Caroline.  You  succeed  to  perfection  :  this  is  indeed  a 
most  interesting  and  conclusive  experiment. 

Emily.  Yet,  Mrs.  B.,  I  cannot  help  thinking,  that  there 
may  perhaps  be  bat  three  distinct  colors  in  the  spectrum,  red, 
yellovv^,  and  blue  ;  and  that  the  four  others  may  consist  of 
two  of  these  colors  blended  together ;  for,  in  painting,  we 
find  that  by  mixing  red  and  yellovv^,  we  produce  orange  ; 
with  different  proportions  of  red  and  blue,  we  make  violet  or 
any  shade  of  purple  ;  and  yellow  and  blue  form  green. 
Now  it  is  very  natural  to  suppose,  that  the  refraction  of  a 
prism  may  not  be  so  perfect  as  to  separate  the  colored  rays  of 
light  completely,  and  that  those  which  are  contiguous  in  or- 
der of  refrangibility  may  encroach  on  each  other,  and  by  mix- 
ing produce  the  intermediate  colors,  orange,  green,  violet, 
and  indigo. 

Mrs.  B.  Your  observation  is,  I  believe,  neither  quite 
wrong,  nor  quite  right.  Dr.  Wollaston,  who  has  refracted 
light  in  a  more  accurate  manner  than  had  been  previously 
done,  by  receiving  a  very  narrow  line  of  light  on  a  prism, 
foiuid  that  it  formed  a  spectrum,  consisting  of  rays  of  four 
colors  only  ;  but  they  were  not  exactly  those  you  have  nam- 
ed as  primitive  colors,  for  the}*  consisted  of  red,  green, 
blue,  and  violet.  A  very  narrow  line  of  yellow  was  visi- 
ble, at  the  limit  of  the  red  and  green,  which  Dr.  Wollaston 
attributed  to  the  overlapping  of  the  edges  of  the  red  and 
green  light. 

Caroline.  But  red  and  green  mixed  together,  do  not  pro- 
duce yellow  ? 

Mrs.  B.  Not  in  painting  ;  but  it  may  be  so  in  the  primi- 
tive rays  of  the  spectrum.  Dr.  Wollaston  observed  that,  by 
increasing  the  breadth  of  the  aperture  by  which  the  line  of 
light  was  admitted,  the  space  occupied  by  each  colored  ray 
in  the  spectrum  was  augmented,  in  proportion  as  each  por- 
tion encroached  on  the  neighboring  color  and  mixed  with  it ; 
so  that  the  intervention  of  orange  and  yellow,  between  the 
red  and  green,  is  owing,  he  supposes,  to  the  mixture  of  these 
two  colors,  and  the  blue  is  blended  on  the  one  side  with  the 
green,  and  on  the  other  with  the  violet,  forming  the  spectrum 
17* 


19S  uN  REFRACTION  AND  C0L0R3> 

as  it  was  originally  observed  by  Sir  Isaac  Ncwloiij  and  which 
I  have  just  shown  you. 

The  rainbow,  which  exhibits  a  series  of  colors  so  analo- 
gous to  those  of  the  spectrum,  is  formed  by  the  refraction  of 
the  sun's  rays  in  their  passage  through  a  shower  of  rain, 
every  drop  of  which  acts  as  a  prism,  in  separating  the  colored 
rays  as  they  pass  through  it. 

Emily,  Pray,  Mrs.  B.,  cannot  the  sun's  rays  be  collected 
to  a  focus  by  a  lens  in  the  same  manner  as  they  are  by  a 
concave  mirror  ? 

Mrs.  B,  No  doubt  the  same  effect  is  produced  by  the  re- 
fraction of  a  lens  as  by  the  reflection  of  a  concave  mirror  : 
in  the  first,  the  rays  pass  through  the  glass  and  converge  to  a 
focus  behind  it  ;  in  the  latter,  they  are  reflected  from  the 
mirror,  and  brought  to  a  focus  before  it.  A  lens,  when  used 
for  the  purpose  of  collecting  the  sun's  rays,  is  called  a  burning 
glass.  The  sun  now  shines  veiy  bright  ;  if  we  let  the  rays 
fa!!  on  this  lens  you  will  perceive  the  focus. 

Emily,  Oh  yes  :  the  point  of  union  of  the  rays  is  very 
luminous.  I  will  hold  a  piece  of  paper  in  the  focus,  and  see 
if  it  will  take  fire.  The  spot  of  light  is  extremely  brilliant, 
but  the  paper  does  not  burn  ? 

Mrs.  B.  Try  a  piece  of  brown  paper  ; — that  you  see 
takes  fire  almost  irnmediatel3\ 

Caroline.  This  is  surprising  :  for  the  light  appeared  to 
shine  more  intensely  on  the  white  than  on  the  brown  paper. 

Mrs.  B.  The  lens  collects  an  equal  number  of  rays  to  a 
focus,  whether  you  hold  the  v/hite  or  the  brown  paper  there  ; 
but  the  white  paper  appears  more  luminous  in  the  focus,  be- 
cause most  of  the  rays,  instead  of  entering  into  the  paper,  are 
leflected by  it  ;  and  this  is  the  reason  that  the  paper  is  not 
burnt  ;  whilst,  on  the  contrary,  the  brown  paper,  which  ab- 
sorbs more  light  than  it  reflects,  soon  becomes  heated  and 
takes  fire. 

Caroline.  This  is  extremely  curious  ;  but  why  should 
brown  paper  absorb  more  rays  than  white  paper  ? 

Mrs.  B.  I  am  far  from  being  able  to  give  a  satisfactory 
answer  to  that  question.  We  can  form  but  mere  conjecture 
on  this  point ;  and  suppose  that  the  tendency  to  absorb,  or 
reflect  rays,  depends  on  the  arrangement  of  the  minute  parti- 
cles of  the  body,  and  that  this  diversity  of  arrangement  ren- 
ders some  bodies  susceptible  of  reflecting  one  colored  ray, 
and  absorbing  the  others ;  whilst  other  bodies  have  a  ten- 


ON  REFRACTION  AND  COLORS-  199 

deiicy  to  reflect  all  the  colors,  and  others  again j  to  absorb 
them  all. 

Emily,  And  how  do  you  know  which  colors  bodies  have 
a  tendency  to  reflect ;  cr  which  to  absorb  ? 

Mrs,  B.  Because  a  body  always  appears  to  be  of  the 
color  which  it  reflects  ;  for,  as  we  see  only  by  reflected  rays^ 
it  can  appear  but  of  the  color  of  those  rays. 

Caroline,  But  we  see  all  bodies  of  their  own  natural 
color,  Mrs.  B.  ;  the  grass  and  trees,  green  ;  the  sky,  blue  ; 
the  flowers,  of  various  hues. 

Mrs,  B,  True  :  but  why  is  the  grass  green  ?  because  it 
absorbs  all  except  the  green  rays  ;  it  is  therefore  these  only 
which  the  grass  and  trees  reflect  to  our  eyes,  and  which 
makes  them  appear  green.  The  sky  and  flovv^ers  in  the  same 
manner,  reflect  the  various  colors  of  which  they  appear  to  us  ; 
the  rose,  the  red  rays  ;  the  violet,  the  blue  ;  the  jonquil,  the 
yellow,  Szc, 

Caroline,  But  these  are  the  permanent  colors  of  the  gi'ass 
and  flowers,  whether  the  sun's  rays  shine  on  them  or  not. 

Mrs,  B.  Whenever  you  see  those  colors,  the  flowers 
must  be  illumined  by  some  light  ;  and  light,  from  whatever 
source  it  proceeds,  is  of  the  same  nature,  composed  of  the 
various  colored  rays,  which  paint  the  grass,  the  flowers,  and 
every  colored  object  in  nature. 

Caroline,  But,  Mrs.  B.,  the  grass  is  green,  and  the  flow- 
ers are  colored,  whether  in  the  dark,  or  exposed  to  the  light  ? 

Mrs,  B,     Why  should  you  think  so  ? 

Caroline,     It  cannot  be  otherwise. 

Mrs,  B,  A  most  philosophical  reason  indeed  !  But,  as  I 
never  saw  them  in  the  dark,  you  will  allow  me  to  dissent 
from  your  opinion. 

Caroline,  What  color  do  you  suppose  them  to  be,  then, 
in  the  dark  ? 

Mrs,  B,  None  at  all ;  or  black,  which  is  the  same  thing. 
You  can  never  see  objects  without  light.  Light  is  composed 
of  colors,  therefore  there  can  be  no  light  without  colors  ;  and 
though  every  object  is  black,  or  without  color  in  the  dark,  it 
becomes  colored,  as  soon  as  it  becomes  visible.  It  is  visible, 
indeed,  but  by  the  colored  rays  which  it  reflects  ;  therefore 
we  can  see  it  only  when  colored. 

Caroline,  All  you  say  seems  very  true  and  I  know  not 
what  to  object  to  it ;  yet  it  appears  at  the  same  time  i»credi- 


200  ON  REFRACTION  AND  COLORS. 

ble  !  What,  Mrs.  B.,  are  we  all  as  black  as  negroes,  in  the 
dark  ?  you  make  me  shudder  at  the  thought. 

Mrs,  B.  Your  vanity  need  not  be  alarmed  at  the  idea,  as 
you  are  certain  of  never  being  seen  in  that  slate. 

Caroline.  That  is  some  consolation,  undoubtedly  !  but 
what  a  melancholy  reflection  it  is,  that  all  nature  which  ap- 
pears so  beautifully  diversified  with  colors  should  be  one 
uniform  mass  of  blackness  ! 

Mrs.  B.  Is  nature  less  pleasing  for  being  colored,  as  well 
as  iliumlned  by  the  rays  of  light ;  and  are  colors  less  beautiful, 
for  being  accidental,  rather  than  essential  properties  of  bodies  ? 

Providence  appears  to  have  decorated  nature  with  the 
enchanting  divei^ity  of  colors  which  we  so  much  admire,  for 
the  sole  purpose  of  beautifying  the  scene,  and  rendering  it  a 
source  of  pleasurable  enjoyment  :  it  is  an  ornament  which 
etnbeliishes  nature  vvhenever  we  behold  her.  What  reason  is 
there  to  regret  that  she  does  not  w^ar  it  when  she  is  invisible  ? 

Emihj.  I  confess,  Mrs.  B.,  that  I  have  had  my  doubts,  as 
well  as  Caroline,  though  she  has  spared  me  the  pains  of  ex- 
pressing them  ;  bat  I  have  just  thought  of  an  experiment, 
which,  if  it  succeeds,  will,  I  am  sure,  satisfy  us  both.  It  is 
certain,  that  we  cannot  see  bodies  in  the  dark,  to  know 
whether  they  have  then  any  color.  But  we  may  place  a 
colored  body  in  a  ray  of  light,  which  has  been  refracted  by  a 
prism ;  and  if  your  theory  is  true,  the  body,  of  whatever 
color  it  naturally  is,  must  appear  of  the  color  of  the  ray  in 
v/hich  it  is  placed  ;  for  since  it  receives  no  other  colored  rays, 
it  can  reflect  no  others. 

Carolbip,.  Oh  !  that  is  an  excellent  thought,  Emily  ;  will 
you  stand  the  test,  Mrs.  B. 

Mrs.  B.  I  consent  :  but  we  must  darken  the  room,  and 
admit  only  the  ray  which  is  to  be  refracted  ;  otherwise,  the 
white  rays  wall  be  reflected  on  the  body  under  trial  from  various 
parts  of  the  room.  With  what  do  you  choose  to  make  the 
experiment  ? 

Caroline.  This  rose :  look  at  it,  Mrs.  B.,  and  tell  me 
whether  it  is  possible  to  deprive  it  of  its  beautiful  color  ? 

Mrs.  B.  We  shall  see. — I  expose  it  first  to  the  red  rays, 
and  the  flower  appears  of  a  more  brilliant  hue  ;  but  observe 
the  green  leaves — 

Caroline.  They  appear  neither  red  nor  green  ;  but  of  a 
dingy  brown  with  a  reddish  glow  i 


ON  REl  RACTiON  AND  COLORS.  201 

Mrs.  B*  They  cannot  be  green,  because  they  have  no 
green  rays  to  reflect  ;  neither  are  they  red,  because  green 
bodies  absorb  most  of  the  red  rays.  But  though  bodies,  from 
the  arrangement  of  their  particles,  have  a  tendency  to  absorb 
some  rays,  and  reflect  others,  yet  it  is  not  natural  to  suppose^ 
that  bodies  are  so  perfectly  uniform  in  their  arrangement,  as 
to  reflect  only  pure  rays  of  one  color,  and  perfectly  absorb  the 
others  ;  it  is  found,  on  the  contrary,  that  a  body  reflects,  in 
great  abundance,  the  rays  w  hich  determine  its  color,  and  the 
others  in  a  greater  or  less  degree,  in  proportion  as  they  are 
nearer  or  further  from  its  own  color,  in  the  order  of  refrangi- 
bility.  The  green  leaves  of  the  rose,  therefore,  will  reflect  a 
few  of  the  red  rays,  which,  blended  with  their  natural  black- 
ness, give  them  that  brown  tinge  ;  if  they  reflected  none  of 
the  red  rays,  they  would  appear  perfectly  black.  Now  I 
shall  hold  the  rose  in  the  blue  rays — 

Caroline,  Oh,  Emily,  Mrs.  B.  is  right  !  look  at  the  rose  : 
it  is  no  longer  red,  but  of  a  dingy  blue  color. 

Emily,  This  is  the  most  wonderful  of  any  thing  we  have 
yet  learnt.  But,  Mrs.  B.,  what  is  the  reason  that  the  green 
leaves  are  of  a  brighter  blue  than  the  rose  ? 

Mrs,  B,  The  green  leaves  reflect  both  blue  and  yellow 
rays,  which  produces  a  green  color.  They  are  now  in  a  col- 
ored ray,  which  they  have  a  tendency  to  reflect  ;  they^ 
therefore,  reflect  more  of  the  blue  ?ays  than  tlie  rose,  (which 
naturally  absorbs  that  color,)  and  will,  of  course,  appear  of  a 
brighter  blue. 

Emihj,  Yet,  in  passing  the  rose  through  the  different  col« 
ors  of  the  spectrum,  the  flower  takes  them  more  readily  than 
the  leaves. 

Mrs,  B,  Because  the  flower  is  of  a  paler  hue.  Bodies 
which  reflect  all  the  rays  are  white  ;  those  which  absorb  them 
all  are  black  :  between  these  extremes,  the  body  appears 
lighter  or  darker,  in  proportion  to  the  quantity  of  rays  they 
reflect  or  absorb.  This  rose  is  of  a  pale  red  :  it  approaches 
nearer  to  white  than  black  ;  it  therefore  reflects  rays  more 
abundantly  than  it  absorbs  them. 

Emily,  But  if  a  rose  has  so  strong  a  tendency  to  reflect 
rays,  I  should  have  imagined  that  it  would  be  of  a  deep  red 
color. 

Mrs,  B.  I  mean  to  say,  that  it  has  a  general  tendency  to 
reflect  rays.  Pale-colored  bodies  reflect  all  the  colored  rays 
to  ^  certain  degree,  which  produces  their  paleness,  approach- 


J02  ON  REFRACTION  AND  COLORS. 

ing  to  whiteness  ;  but  one  color  they  reflect  more  than  the 
rest  ;  this  predominates  over  the  white,  and  determines  the 
color  of  the  body.  Since,  then,  bodies  of  a  pale  color  in 
some  degree  reflect  all  the  rays  of  light,  in  passing  through 
the  various  colors  of  the  spectrum,  they  will  reflect  them  all 
with  tolerable  brilliancy  ;  but  will  appear  most  vivid  in  the 
ray  of  their  natural  color.  The  green  leaves,  on  the  contra- 
ry, are  of  a  dark  color,  bearing  a  stronger  resemblance  to 
black,  than  to  white  ;  they  have,  therefore,  a  greater  tenden- 
cy to  absorb,  than  to  reflect  rays  ;  and  reflecting  very  few 
of  any  but  the  blue  and*yellow  rays,  they  will  appear  dingy 
in  passing  through  the  other  colors  of  the  spectrum. 

Caroline,  They  must,  however,  reflect  great  quantities  of 
the  green  rays,  to  produce  so  deep  a  color. 

^  rs.  B,  Deepness  or  darkness  of  color  proceeds  rather 
from  a  deficiency  than  an  abundance  of  reflected  rays.  Re- 
member that  bodies  are,  of  themselves,  black  ;  and  if  a  body 
reflects  only  a  few  green  rays,  it  will  appear  of  a  dark  green  ; 
it  is  the  brightness  and  intensity  of  the  color  w  hich  show  that 
a  great  quantity  of  rays  are  reflected. 

Emily,  A  white  body,  then,  which  reflects  all  the  rays^ 
will  appear  equally  bright  in  all  the  colors  of  the  spectrum. 

Mrs.  B,  Certainly  ;  and  this  is  easily  proved  by  passing 
a  sheet  of  white  paper  through  the  rays  of  the  spectrum. 

Caroline,  What  is  the  reason  that  blue  often  appears 
green  by  candle-lis^ht  ? 

Mrs.  B.  The  light  of  a  candle  is  not  so  pure  as  that  of 
the  sun  ;  it  has  a  yellowish  tinge,  and  when  refracted  by  the 
prism,  the  yellow  rays  predominate  ;  and  as  blue  bodies 
reflect  the  yellow  rays  in  the  next  proportion,  (being  next  in 
order  of  refransfibility,)  the  superabundance  of  yellow  rays 
gives  to  blue  bodies  a  greenish  hue. 

Caroline,  Candle-light  must  then  give  to  all  bodies  a 
yellowish  tinge,  from  the  excess  of  yellow  rays  ;  and  yet  it  is 
a  common  remark,  that  people  of  a  sallow  complexion  appear 
fairer  or  whiter  by  candle-light. 

Mrs.  B,  The  yellow  cast  of  their  complexion  is  not  so 
striking,  when  every  object  has  a  yellow  tinge. 

Emily,     Pray,  w^hy  does  the  sun  appear  red  through  a  fog? 

Mrs.  B,  It  is  supposed  to  be  owing  to  the  red  rays  having 
a  greater  momentum,  which  gives  them  power  to  traverse  so 
dense  an  atmosphere.  For  the  same  reason,  the  sun  generally 
appears  red  at  rising  and  sotting  :  as  the  increased  quantity 


ON  REFRACTION  AND  COLORS.  203 

of  atmosphere^  which  the  oblique  rays  must  traverse,  loaded 
with  the  mists  and  vapors  which  are  usuall}^  formed  at  those 
times  prevents  the  other  rays  from  reaching  us. 

Caroline.     And,  pray,  why  are  the  skies  of  a  blue  color  ? 

Mrs,  B,  You  should  rather  say,  the  atmosphere  ;  for  the 
sky  is  a  very  vague  term,  the  meaning  of  which  it  would  be 
difficult  to  define  philosophically. 

Caroline,  But  the  color  of  the  atmosphere  should  be 
white,  since  all  the  ra3^s  traverse  it  in  their  passage  to  the 
earth. 

Mrs.  B.  Do  not  forget  that  we  see  none  of  the  rays 
which  pass  from  the  sun  to  the  earth,  excepting  those  which 
meet  our  eyes  ;  and  this  happens  only  if  we  look  at  the  sun, 
and  thus  intercept  the  ra3^s,  in  which  case,  you  know,  the  sun 
appears  white.  The  atmosphere  is  a  transparent  medium, 
through  which  the  sun^s  rays  pass  freely  to  the  earth  ;  but 
when  reflected  back  into  the  atmosphere,  their  momentum  is 
considerably  diminished  ;  and  they  have  not  all  of  them 
power  to  traverse  it  a  second  time.  The  momentum  of  the 
blue  rays  is  least  ;  these,  therefore,  are  the  most  impeded  in 
their  return,  and  are  chiefly  reflected  by  the  atmosphere  :  this 
reflection  is  performed  in  every  possible  direction  ;  so  that 
whenever  we  look  at  the  atmosphere,  some  of  these  rays  fall 
upon  our  eyes  ;  hence  we  see  the  air  of  a  blue  color.  If  the 
atmosphere  did  not  reflect  any  rays,  though  the  objects  on  the 
surface  of  the  earth  would  be  illumined,  the  skies  would 
appear  perfectly  black. 

Caroline.  Oh,  how  melancholy  that  would  be  ;  and  how 
pernicious  to  the  sight,  to  be  constantly  viewing  bright  objects 
against  a  black  sky.  But  what  is  the  reason  that  bodies  often 
change  their  color  ;  as  leaves  which  wither  in  autumn,  or  a 
spot  of  ink  which  produces  an  iron-mould  on  linen  ? 

Mrs.  B.  It  arises  from  some  chemical  change,  which 
takes  place  in  the  internal  arrangement  of  the  parts,  by  which 
they  lose  their  tendency  to  reflect  certain  colors,  and  acquire 
the  power  of  reflecting  others.  A  withered  leaf  thus  no  lon- 
ger reflects  the  blue  rays  ;  it  appears,  therefore,  yellow,  or 
has  a  slight  tendency  to  reflect  several  rays  which  produce  a 
dingy  brown  color. 

An  ink-spot  on  linen  at  first  absorbs  all  the  rays ;  but^ 
exposed  to  the  air,  it  undergoes  a  chemical  change,  and  the 
spot  partially  regains  its  tendency  to  reflect  colors^  but  with 


204  ON  REFRACTION  AND  COLORS. 

a  preference  to  reflect  the  yellow  rays^  and  such  is  the  color 
of  the  iron-mould. 

Emilij,  Bodies,  then,  far  from  being  of  the  color  which 
they  appear  to  possess,  are  of  that  color  which  they  have  the 
greatest  aversion  to,  w^hich  they  will  not  incorporate  with, 
but  reject  and  drive  from  them. 

Mrs,  B,  It  certainly  is  so  ;  though  I  scarcely  dare  ven- 
ture to  advance  such  an  opinion  whilst  Caroline  is  contem- 
plating her  beautiful  rose. 

Caroline.  My  poor  rose  !  you  are  not  satisfied  with 
depriving  it  of  color,  but  even  make  it  have  an  aversion  to  it ; 
and  I  am  unable  to  contradict  you. 

Emily,  Since  dark  bodies  absorb  more  solar  rays  than 
light  ones,  the  former  should  sooner  be  heated  if  exposed  to 
the  sun  ? 

Mrs,  B,  And  they  are  found  by  experience  to  be  so. 
Have  you  never  observed  a  black  dress  to  be  warmer  than  a 
white  one  ? 

Emily,  Yes,  and  a  white  one  more  dazzling  :  the  black 
is  heated  by  absorbing  the  rays,  the  white  dazzling  by  reflect- 
ing them. 

Caroline,  And  this  was  the  reason  that  the  brown  paper 
was  burnt  in  the  focus  of  the  lens,  whilst  the  white  paper 
exhibited  the  most  luminous  spot,  but  did  not  take  fire. 

Mrs,  B,  It  was  so.  It  is  now  full  time  to  conclude  our 
lesson.  At  our  next  meeting,  I  shall  give  you  ^  (description 
of  the  eye. 


^^•<' 


TLATE    TTT 


t3STERSATI0N  XVH. 


OPTICS. 

ON   THE   STRUCTURE  OF  THE   EYE,   AND   OPTICAL 
INSTRUMENTS. 

Description  of  the  Eye  ;  Of  the  Image  on  the  Retina  ; 
Refraction  of  the  Humors  of  the  Eye  ;  Of  the  Use  of 
Spectacles;  Of  the  Single  Microscope  ;  Of  the  Double 
Microscope ;  Of  the  Solar  Microscope ;  Magic  LaU' 
thorn  ;  Refracting  Telescope  ;  Refecting  Telescope, 


MRS.B. 

The  body  of  the  eye  is  of  a  spherical  form  :  (fig.  1.  plate 
XXL)  it  has  two  membranous  coverings  ;  the  external  one^ 
a  a  Oy  is  called  the  sclerotica;  this  has  a  projection  in  that 
part  of  the  eye  which  is  exposed  to  view,  b  b,  which  is  called 
the  cornea,  because,  when  dried,  it  has  nearly  the  consist- 
ence of  very  fine  horn,  and  is  sufficiently  transparent  for  the 
light  to  obtain  free  passage  through  it. 

The  second  membrane  which  lines  the  cornea,  and  envel- 
opes the  eye,  is  called  the  choroid,  c  c  c  ;  this  has  an  open- 
ing in  front,  just  beneath  the  cornea,  which  forms  the  pupil, 
d  5,  through  which  the  rays  of  light  pass  into  the  eve.  The 
pupil  is  surrounded  by  a  colored  border,  called  the"  iris,  e  e, 
which  by  its  muscular  motion,  always  preserves  the  pupil  of 
a  circular  form,  whether  it  is  expanded  in  the  dark,  or  con- 
tracted by  a  strong  light.  This  you  will  understand  better 
by  examining  fig.  2. 

Emily.  1  did  not  know  that  the  pupil  was  susceptible  of 
varying  its  dimensions. 

Mrs,  B.     The  construction  of  the  eye  is  so  admirable,  that 
it  is  capable  of  adapting  itself,  more  or  less,  to  the  circum- 
18 


20G  OPTICS. 

stances  in  which  it  is  placed.  In  a  faint  light  the  pupil  di- 
lates so  as  to  receive  an  additional  quantity  of  rays,  and  in  a 
strong  light  it  contracts,  in  order  to  prevent  the  intensity  of 
the  light  from  injuring  the  optic  nerve.  Observe  Emily's 
eyes,  as  she  sits  looking  towards  the  windows  :  her  pupils 
appear  very  small,  and  the  iris  large.  Now,  Emily,  turn 
from  the  light,  and  cover  your  eyes  with  your  hand,  so  as 
entirely  to  exclude  it  for  a  few  moments. 

Caroline.  How  very  much  the  pupils  of  her  eyes  are 
now  enlarged,  and  the  iris  diminished.  This  is,  no  doubt, 
the  reason  why  the  eyes  suffer  pain,  when  from  darkness 
they  suddenly  come  into  a  strong  light  ;  for  the  pupil  being 
dilated,  a  quantity  of  rays  must  rush  in  before  it  has  time  to 
contract. 

Emily,  And  when  we  go  from  a  strong  light  into  obscu- 
rity, we  at  first  imagine  ourselves  in  total  darkness ;  for  a 
sufficient  number  of  rays  cannot  gain  admittance  into  the 
contracted  pupil,  to  enable  us  to  distinguish  objects  :  but  in  a 
iew  minutes  it  dilates,  and  we  clearly  perceive  objects  which 
were  before  invisible. 

Mrs.  B.  It  is  just  so.  The  choroid  c  c,  is  imbued  with  a 
black  liquor  which  serves  to  absorb  all  the  rays  that  are 
irregularly  reflected,  and  to  convert  the  body  of  the  eye  into 
a  more  perfect  camera  obscura.  When  the  pupil  is  expand- 
ed to  its  utmost  extent,  it  is  capable  of  admitting  ten  times 
the  quantity  of  light  that  it  does  when  most  contracted.  In 
cats,  and  animals  which  are  said  to  see  in  the  dark,  the  power 
of  dilatation  and  contraction  of  the  pupil  is  still  greater  :  it  is 
computed  that  their  pupils  may  receive  one  hundred  times 
more  light  at  one  time  than  at  another. 

Within  these  coverings  of  the  eye-ball  are  contained  three 
transparent  substances,  called  humors.  The  first  occupies 
the  space  immediately  behind  the  cornea,  and  is  called  the 
aqueous  humor,  ff^  from  its  liquidity  and  its  resemblance  to 
w  ater.  Beyond  this  is  situated  the  crystalline  humor,  g  g, 
so  called  from  its  clearness  and  transparency  :  it  has  the 
form  of  a  lens,  and  refracts  the  rays  of  light  in  a  greater  de- 
gree of  perfection  than  any  that  have  been  constructed  by- 
art  :  it  is  attached  by  two  muscles,  m  m,  to  each  side  of  the 
choroid.  The  back  part  of  the  eye,  between  the  crystalline 
humor  and  the  retina,  is  filled  by  the  vitreous  humor,  h  k, 
which  derives  its  name  from  a  resemblance  it  is  supposed  to 
bear  to  elass  or  vitrified  substances. 


OPTICS.  207 

The  membranous  coverings  of  the  eye  are  intended  chiefly 
for  the  preservation  of  the  retina,  i  ?,  which  is  by  far  the  most 
important  part  of  the  eye,  as  it  is  that  which  receives  the 
impression  of  the  objects  of  sight,  and  conveys  it  to  the  mind. 
The  retina  consists  of  an  expansion  of  the  optic  nerve,  of  a 
most  perfect  whiteness :  it  proceeds  from  the  brain,  enters 
the  eye,  at  w,  on  the  side  next  the  nose,  and  is  finely  spread 
over  the  interior  surface  of  the  choroid. 

The  rays  of  light  which  enter  the  eye  by  the  pupil  are 
refracted  by  the  several  humors  in  their  passage  through  them, 
and  unite  in  a  focus  on  the  retina. 

Caroline.  I  do  not  understand  the  use  of  these  refracting 
humors  :  the  image  of  objects  is  represented  in  the  camera 
obscura,  wdthout  any  such  assistance. 

IMrs,  B.  That  is  true  ;  but  the  reoresentation  would  be 
much  more  strong  and  distinct,  if  we  enlarge  the  opening  of 
the  camera  obscura,  and  received  tlie  rays  into  it  through  a 
lens. 

I  have  told  you  that  rays  proceed  from  bodies  in  all  possi- 
bFe  directions.  We  must,  therefore,  consider  every  part  of 
an  object  which  sends  rays  to  our  eyes,  as  points  from  which 
the  rays  divergfe,  as  from  a  centre. 

Emilf/.  These  divergent  rays,  issuing*  from  a  single  point, 
I  believe  you  told  us,  were  called  a  pencil  of  rays  ? 

Mrs.  B.  Yes.  Now,  divergent  rays,  on  entering  the 
pupil,  do  not  cross  each  other  ;  the  pupil,  however,  is  suffi- 
ciently large  to  admit  a  small  pencil  of  them  ;  and  these,  if 
not  refracted  to  a  focus  by  the  humors,  would  continue  diverg- 
ing after  they  had  passed  the  pupil,  would  fall  dispersed  upon 
the  retina,  and  thus  the  image  of  a  single  point  would  be 
expanded  over  a  large  portion  of  the  retina.  The  divergent 
rays  from  every  other  point  of  the  object  would  be  spread 
over  a  similar  extent  of  space,  and  would  interfere  and  be 
confounded  with  the  first ;  so  that  no  distinct  image  could  be 
formed,  and  the  retina  would  represent  total  confusion  both 
of  figure  and  color.  Fig.  3.  represents  two  pencils  of  rays 
issuing  from  two  points  of  the  tree  A  B,  and  entering  the 
pupil  C,  refracted  by  the  crystalline  humor  D,  and  forming 
distinct  images  of  the  spot  they  proceed  from,  on  the  retina  at 
a  b.  Fig.  4.  differs  from  the  preceding,  merely  from  not 
being  supplied  with  a  lens  ;  in  consequence  of  which  the 
pencils  of  rays  are  not  refracted  to  a  focus,  and  no  distinct 

na<re  is  formed  on  the  retina.     T  have  delineated  only  the 


208  OPTICS. 

rays  issuing  from  two  points  of  an  object,  and  distinguished 
tlie  two  pencils  in  fig.  4.  by  describing  one  of  them  with  dot- 
ted lines  ;  the  interference  of  these  two  pencils  of  rays  on  the 
retina  will  enable  you  to  form  an  idea  of  the  confusion  which 
would  arise,  from  thousands  and  millions  of  points  at  the  same 
instant  pouring  their  divergent  rays  upon  the  retina. 

Emily.  True  ;  but  I  do  not  yet  well  understand  how  the 
refracting  humors  remedy  this  imperfection. 

Mrs.  B.  The  refraction  of  these  several  humors  unite  the 
w^hole  of  a  pencil  of  rays,  proceeding  from  any  one  point  of  an 
object,  to  a  corresponding  point  on  the  retina,  and  the  image 
is  thus  rendered  distinct  and  strong.  If  you  conceive,  in  fig. 
3.,  ev-ery  point  of  the  tree  to  send  forth  a  pencil  of  rays  simi- 
lar to  those,  A  B,  every  part  of  the  tree  will  be  as  accurately 
represented  on  the  retina  as  the  points  a  h. 

Emily.  How  admirably,  how  wonderfully,  this  is  con« 
trived  ! 

Caroline.  But  since  the  eye  requires  refracting  humors  in 
order  to  have  a  distinct  representation  formed  on  the  retina, 
why  is  not  the  same  refraction  necessary  for  the  image  formed 
in  the  camera  obscura. 

Mrs.  B.  Because  the  aperture  through  which  we  received 
the  rays  into  the  camera  obscura  is  extremely  small  ;  so  that 
but  very  few  of  the  rays  diverging  from  a  point  gain  admit- 
tance ;  but  we  will  now  enlarge  the  aperture,  and  furnish  it 
with  a  lens,  and  you  will  find  the  landscape  to  be  more 
peifecrly  represented. 

Caroline.  How  obscure  a.id  confused  the  image  is  now 
that  you  have  enlarged  the  opening,  without  putting  in  the 
lens. 

Mrs.  B.  Such,  or  ver}^  similar  would  be  the  representation 
on  the  retina,  unassisted  by  the  refracting  humors.  But  see 
what  a  difference  is  produced  by  the  introduction  of  the  lens, 
which  collects  each  pencil  of  divergent  rays  into  their  several 
foci. 

Caroline.  The  alteration  is  wonderful  :  the  representation 
is  more  clear,  vivid,  and  beautiful  than  ever. 

Mrs.  B.  You  will  now  be  able  to  understand  the  nature 
of  that  imperfection  of  sight,  which  arises  from  the  eyes  being 
too  prominent.  In  such  cases,  the  crystalline  humor,  D, 
(fig.  5.)  being  extreme^  convex,  refracts  the  rays  too  much, 
and  collects  a  pencil,  proceeding  from  the  object  A  B,  into  a 
focus,  F,  before  they  reach  the  retina.     From  this  focus  tlie* 


Tiq.  4 


OPTICS.  209 

rays  proceed  diverging^  and  consequently  form  a  very  confused 
image  on  the  retina  at  a  b.  This  is  the  defect  of  short-sighted 
people. 

Emily,  I  understand  it  perfectly.  But  why  is  this  defect 
remedied  by  bringing  the  object  nearer  to  the  eye^  as  we  find 
to  be  the  case  with  short-sighted  people  ? 

Mrs.  B,  The  nearer  you  bring  an  object  to  your  eye  the 
more  divergent  the  rays  fall  upon  the  crystalline  humor,  and 
they  are  consequently  not  so  soon  converged  to  a  focus  :  this 
focus  therefore,  either  falls  upon  the  retina,  or  at  least  ap- 
proaches nearer  to  it,  and  the  object  is  proportionally  distinct, 
as  in  fig.  6. 

Emily.  The  nearer,  then,  you  bring  an  object  to  a  lens 
the  further  the  image  recedes  behind  it. 

Mrs.  B.  Certainly.  But  short-sighted  persons  have 
another  resource  for  objects  which  they  cannot  approach  to 
their  eyes  ;  this  is  to  place  a  concave  lens,  C  D,  (fig.  1.  plate 
XXII.)  before  the  eye,  in  order  to  increase  the  divergence  of 
the  rays.  The  effect  of  a  concave  lens  is,  you  know,  exactly 
the  reverse  of  a  convex  one  :  it  renders  parallel  rays  divergent 
and  those  which  are  already  divergent,  still  more  so.  By  the 
assistance  of  such  glasses,  therefore,  the  rays  from  a  distant 
object  fall  on  the  pupil,  as  divergent  as  those  from  a  less  dis- 
tant object ;  and,  with  short-sighted  people,  they  throw  the 
image  of  a  distant  object  back  as  far  as  the  retina. 

Caroline.     This  is  an  excellent  contrivance,  indeed. 

Mrs.  B.  And  tell  me,  what  remedy  would  you  devise  for 
such  persons  as  have  a  contrary  defect  in  their  sight ;  that  is 
to  say,  in  whom  the  crystalline  humor,  being  too  flat,  does  not 
refract  the  rays  sufficiently,  so  that  they  reach  the  retina  before 
they  are  converged  to  a  point  ? 

Caroline.  I  suppose  that  a  contrary  remedy  must  be  ap- 
plied to  this  defect ;  that  is  to  say,  a  convex  lens,  L  M,  fig. 
2.,  to  make  up  for  the  deficiency  of  convexity  of  the  crystal- 
line humor,  O  P.  For  the  convex  lens  would  bring  the  rays 
nearer  together,  so  that  they  would  fall  either  less  divergent, 
or  parallel  on  the  crystalline  humor  ;  and,  by  being  sooner 
converged  to  a  focus,  would  fall  on  the  retina. 

Mrs.  B.  Very  well,  Caroline.  This  is  the  reason  why 
elderly  people,  the  humors  of  whose  eyes  are  decayed  by  age, 
are  under  the  necessity  of  using  convex  spectacles.  And 
when  deprived  of  that  resource,  they  hold  the  object  at  a 

18* 


;o  OPTICS. 

distance  from  their  eyes,  as  in  fig,  4,  in  order  to  bring  the  locus 
forwarder. 

Caroline.  I  have  often  been  surprized,  when  my  grand- 
father reads  without  his  spectacles,  to  see  him  hold  the  book 
at  a  considerable  distance  from  his  eyes.  But  I  now  under- 
stand it  ;  for  the  more  distant  the  object  is  from  the  crystal- 
line, the  nearer  the  image  will  be  to  it. 

Emily,  I  comprehend  the  nature  of  these  two  opposite 
defects  very  well  ;  but  I  cannot  now  conceive,  how  any  sight 
can  be  perfect :  for  if  the  crystalline  humor  is  of  a  proper 
degree  of  convexity,  to  bring  the  image  of  distant  objects  to  a 
focus  on  the  retina,  it  will  not  represent  near  objects,  distinct- 
ly ;  and  if,  on  the  contrary,  it  is  adapted  to  give  a  clear  image 
of  near  objects,  it  will  produce  a  very  imperfect  one  of  distant 
objects. 

Mrs,  B,  Your  observation  is  very  good,  Emily  ;  and  it  is> 
true,  that  every  person  would  be  subject  to  one  of  these  two 
defects,  if  we  had  it  not  in  our  power  to  increase  or  diminish 
the  convexity  of  the  crystalline  humor,  and  to  project  it 
towards,  or  draw  it  back  from  the  object,  as  circumstances 
require.  In  a  young  well-constructed  eye,  the  two  muscles 
to  which  the  crystalline  humor  is  attached  have  so  perfect  a 
command  over  it,  that  the  focus  of  the  rays  constantly  falls  on 
the  retina,  and  an  image  is  formed  equally  distinct  both  of 
distant  objects  and  of  those  which  are  near. 

Caroline,  In  the  eyes  of  fishes,  which  are  the  only  eyes  I 
have  ever  seen  separate  from  the  head,  the  cornea  does  not 
protrude,  in  that  part  of  the  eye  which  is  exposed  to  view. 

Mrs,  B,  The  cornea  of  the  eye  of  a  iish  is  not  more  convex 
ihan  the  rest  of  the  ball  of  the  eye  ;  but  to  supply  this  defi- 
ciency, their  crystalline  humor  is  spherical,  and  refracts  the 
rays  so  much,  that  it  does  not  require  the  assistance  of  the 
cornea  to  bring  them  to  a  focus  on  the  retina. 

Emily,  Pray,  w^hat  is  the  reason  that  we  cannot  see  an 
object  distinctly,  if  we  approach  it  very  near  to  the  eye  ? 

Mrs,  B,  Because  the  rays  fall  on  the  crystalline  humor 
too  divergent  to  be  refracted  to  a  focus  on  the  retina  ;  the 
confusion,  therefore,  arising  from  viewing  an  object  too  near 
the  eye,  is  similar  to  that  which  proceeds  from  a  flattened 
(^rystalline  humor  ;  the  rays  reach  the  retina  before  they  are 
^^ollected  to  a  focus,  {Jig,  4.)  If  it  were  not  for  this  imper- 
'■^<tion.  we  should  be  able  to  see  and  distinguish  the  parts  of 


OPTICS.  2il 

objects^  which  are  now  invisible  to  us  from  their  minuteness  ; 
for  could  we  approach  them  very  near  the  eye,  their  image 
on  the  retina  would  be  so  much  magnified  as  to  render  them 
visible. 

Emily,  And  could  there  be  no  contrivance  to  convey  the 
rays  of  objects  viewed  close  to  the  eye,  so  that  they  should  be 
refracted  to  a  focus  on  the  retina  ? 

Mrs,  B,  The  microscope  is  constructed  for  this  purpose. 
The  single  microscope  (fig.  5.)  consists  simply  of  a  convex 
lens,  commonly  called  a  magnifying  glass  ;  in  the  focus  of 
which  the  object  is  placed,  and  through  which  it  is  viev/ed  : 
by  this  means,  you  are  enabled  to  approach  your  eye  very 
near  the  object,  for  the  lens  A  B,  by  diminishing  the  diver- 
gence of  the  rays,  before  they  enter  the  pupil  C,  makes  them 
fall  parallel  on  the  crystalline  humor  D,  by  which  they  are 
refracted  to  a  focus  on  the  retina,  at  R  R. 

Emily,  This  is  a  most  admirable  invention,  and  nothing 
can  be  more  simple,  for  the  lens  magnifies  the  object  merely 
by  allowing  us  to  bring  it  nearer  to  the  eye. 

Mrs,  B,  Those  lenses,  therefore,  which  have  the  shortest 
focus  will  magnify  the  object  most,  because  they  enable  us  to 
bring  the  object  nearest  to  the  eye. 

Emily,  But  a  lens,  that  has  the  shortest  focus,  is  most 
bulging  or  convex ;  and  the  protuberance  of  the  lens  will 
prevent  the  eye  from  approachhig  very  near  to  the  object. 

Mrs,  B,  This  is  remedied  by  making  the  lens  extremel}^ 
small  :  it  may  then  be  spherical  without  occupying  much 
space,  and  thus  unite  the  advantages  of  a  short  focus,  and  of 
allowing  the  eye  to  approach  the  object. 

Caroline,  We  have  a  microscope  at  home,  which  is  a 
much  more  complicated  instrument  than  that  you  liave  des- 
cribed. 

Mrs.  B,  It  is  a  double  microscope  (fig.  6.),  in  which  you 
see  not  the  object  A  B,  but  a  magnified  image  of  it,  a  b.  In 
this  microscope,  two  lenses  are  employed,  the  one  L  M,  for 
the  purpose  of  magnifying  the  object,  is  called  the  object 
glass  ;  the  other  N  O,  acts  on  the  principle  of  the  single  mi- 
croscope, and  is  called  the  eye-glass. 

There  is  another  kind  of  microscope,  called  the  solar  mi- 
croscope, which  is  the  most  wonderful  from  its  great  magni- 
fying power  ;  in  this  we  also  view  an  image  formed  by  a 
lens,  not  the  object  itself.  As  the  sun  shines,  I  can  show  you 
die  effect  of  this  microscope  :  but  for  this  purpose,  we  must 


212  OPTICS. 

close  the  shutters,  and  admit  only  a  small  portion  of  light, 
through  the  hole  in  the  window-shutter,  which  we  used  for 
the  camera  obscura.  We  shall  now  place  the  object  A  B, 
(plate  XXIII.  fig,  1.)  which  is  a  small  insect,  before  the  lens 
C  D,  and  nearly  at  its  focus  ;  the  image  E  F,  will  then  be 
represented  on  the  opposite  wall  in  the  same  manner  as  the 
landscape  was  in  the  camera  obscura  ;  with  this  difference, 
that  it  will  be  magnified,  instead  of  being  diminished.  I 
shall  leave  you  to  account  for  this,  by  examining  the  figure. 

Emily,  1  see  it  at  once.  The  image  E  F  is  magnified, 
because  it  is  farther  from  the  lens,  than  the  object  A  B ; 
while  the  representation  of  the  landscape  was  diminished 
because  it  was  nearer  the  lens,  than  the  landscape  was.  A 
lens,  then,  answers  the  purpose  equally  well,  either  for  mag- 
nifying or  diminishing  objects  ? 

Mrs,  B,  Yes  :  if  you  wish  to  magnify  the  image,  you 
place  the  object  near  the  focus  of  the  lens  ;  if  you  wish  to 
produce  a  diminished  image,  you  place  the  object  at  a  dis- 
tance from  the  lens,  in  order  that  the  image  may  be  formed 
in,  or  near  the  focus. 

Caroline.  The  magnifying  power  of  this  microscope,  is 
prodigious,  but  the  indistinctness  of  the  image  for  want  of 
light,  is  a  great  imperfection.  AVould  it  not  be  clearer,  if  the 
opening  in  the  shutter  were  enlarged,  so  as  to  admit  more 
light. 

Mrs,  B,  If  the  whole  of  the  light  admitted  does  not  fall 
upon  the  object,  the  effect  will  only  be  to  make  the  room 
lighter,  and  the  image  consequently  less  distinct. 

Emily,  But  could  you  not  by  means  of  another  lens  bring 
a  large  pencil  of  rays  to  a  focus  on  the  object,  and  thus  con- 
centrate the  whole  of  the  light  admitted  upon  it  ? 

Mrs,  B,  Very  well.  We  shall  enlarge  the  opening  and 
place  the  lens  X  Y  (fig,  2.)  in  it,  to  converge  the  rays  to  a 
focus  on  the  object  A  B.  There  is  but  one  thing  more  wan- 
ting to  complete  the  solar  micr-oscope,  which  I  shall  leave  to 
Caroline's  sagacity  to  discover. 

Caroline,  Our  microscope  has  a  small  mirror  attached  to 
it,  upon  a  moveable  joint,  which  can  be  so  adjusted  as  to 
receive  the  sun^s  rays,  and  reflect  them  upon  the  object ;  if  a 
similar  mirror  were  placed  to  reflect  light  upon  the  lens, 
would  it  not  be  a  means  of  illuminating  the  object  more 
perfectly. 

Mrs,  B.     You  are  quite  right.     P  Q  (fig.  2.)  is  a  small 


j'LATi:  xxm. 


c       ^e-  * 


OPTICS.  213 

mirror  placed  on  the  outside  of  the  window  shutter^  which 
receives  the  incident  rays  S  S,  and  reflects  them  on  the  lens 
X  Y.  Now  that  we  have  completed  the  apparatus  let  us 
examine  the  mites  on  this  piece  of  cheese^  which  I  place  near 
the  focus  of  the  lens. 

Caroline.  Oh,  how  much  more  distinct  the  image  now  is, 
and  how  wonderfully  magnified  ;  the  mites  on  the  cheese  look 
Uke  a  drove  of  pigs  scrambling  over  rocks  ? 

Emily,  I  never  saw  any  thing  so  curious.  Now,  an 
immense  piece  of  cheese  has  fallen :  one  would  imagine  it 
an  earthquake  :  some  of  the  poor  mites  must  have  been 
crushed  ;  how  fast  they  run, — they  absolutely  seem  to  gallop. 

But  this  microscope  can  be  used  only  for  transparent  ob- 
jects ;  as  the  light  must  pass  through  them  to  form  the  image 
on  the  wall. 

Mrs.  B.  Very  minute  objects,  such  as  are  viewed  in  a 
microscope,  are  generally  transparent  ;  but  when  opaque  ob- 
jects are  to  be  exhibited,  a  mirror  M  N  (fig.  3.)  is  used  to 
reflect  the  light  on  the  side  of  the  object  next  the  wall  :  the 
image  is  then  formed  by  light  reflected  from  the  object,  instead 
of  being  transmitted  through  it. 

Einiiy,  Pray  is  not  a  magic  lanthorn  constructed  on  the 
same  principles  ?'* 

Mrs.  B*  Yes  ;  with  this  difference  that  the  light  is  sup- 
plied by  a  lamp,  instead  of  the  sun. 

The  micro.'^cope  is  an  excellent  invention,  to  enable  us  to 
see  and  distinguish  objects,  which  are  too  small  to  be  visible  to 
the  naked  eye.  But  there  are  objects  which,  though  not 
really  small,  appear  so  to  us,  fi'om  their  distance  ;  to  these  we 
cannot  apply  the  same  remedy ;  for  when  a  house  is  so  far 
distant,  as  to  be  seen  under  the  same  angle  as  a  mite  which  is 
close  to  us,  the  effect  produced  on  the  retina  is  the  same  :  the 
angle  it  subtends  is  not  large  enough  for  it  to  form  a  distinct 
image  on  the  retina. 

Emily.  Since  it  is  impossible,  in  this  case,  to  approach  the 
object  to  the  eye,  cannot  v/e  by  means  of  a  lens  bring  an  image 
of  it  nearer  to  us  ? 

Mrs.  B.  Yes  ;  but  then  the  object  being  very  distant 
from  the  focus  of  the  lens,  the  image  weald  be  too  small  to  be 
visible  to  the  naked  eye. 

*  The  ma£^ic  lanthorn  is  an  instrument  used  for  magnifying  paintings  on 
glass,  Riu!  throwing-  their  images  upon  a  white  screen  in  a  darkene(t 
uhambi^r. 


J 14  OPTICS. 

Emily,  Then,  why  not  look  at  the  image  through  another 
lens,  which  will  act  as  a  microscope,  enable  us  to  bring  the 
image  close  to  the  eye,  and  thus  render  it  visible  ? 

Mrs,  B.  Very  well,  Emily  ;  I  congratulate  you  on  hav- 
ing invented  a  telescope.  In  figure  4,  the  lens  C  D,  forms  an 
image  E  F,  of  the  object  A  B  ;  and  the  lens  X  Y,  serves  the 
purpose  of  magnifying  that  image ;  and  this  is  all  that  is 
required  in  a  common  refracting  telescope. 

Emily.  But  in  fig.  4,  the  image  is  not  inverted  on  the 
retina,  as  objects  usually  are  :  it  should  therefore  appear  to 
us  inverted  ;  and  that  is  not  the  case  in  the  telescopes  I  have 
looked  through. 

Mrs,  B.  When  it  is  necessary  to  represent  the  image 
erect,  two  other  lenses  are  required  ;  by  which  means  a  second 
image  is  formed,  the  reverse  of  the  first  and  consequently 
upright.  These  additional  glasses  are  used  to  view  terres- 
trial objects  ;  for  no  inconvenience  arises  from  seeing  the 
celestial  bodies  inverted. 

Emily.  The  difference  between  a  microscope  and  a  teles- 
cope seems  to  be  this  : — a  microscope  produces  a  magnified 
image,  because  the  object  is  nearest  the  lens  ;  ai  d  a  telescope 
produces  a  diminished  image,  because  the  object  is  furthest 
from  the  lens. 

Mrs.  B.  Your  observation  applies  only  to  the  lens  C  D, 
or  object  glass,  which  serves  to  bring  an  image  of  the  object 
nearer  the  eye  ;  for  the  lens  X  Y,  or  eye-glass  is,  in  fact,  a 
microscope,  as  its  purpose  is  to  magnify  the  image. 

When  a  very  great  magnifying  power  is  required,  teles- 
copes are  constructed  with  concave  mirrors,  instead  of  lenses. 
Concave  mirrors,  you  know,  produce  by  reflection,  an  effect 
similar  to  that  of  convex  lenses  by  refraction.  In  reflecting 
telescopes,  therefore,  mirrors  are  used  in  order  to  bring  the. 
image  nearer  the  eye  ;  and  a  lens  or  eye-glass  the  same  as  in 
tJie  refracting  telescope  to  magnify  tlie  image. 

The  advantage  of  the  reflecting  telescope  is,  that  mirrors 
whose  focus  is  six  feet  will  magnify  as  much  as  lenses  of  a 
hundred  feet. 

Caroline.  But  I  thought  it  was  the  eye-glass  only  which 
magnified  the  image  ;  and  that  the  other  lens  served  to  bring 
a  diminished  image  nearer  to  the  eye. 

Mrs.  B.  The  image  is  diminished  in  comparison  to  the 
object,  it  is  true  ;  but  it  is  magnified  if  you  compare  it  to  the 
dimensions  of  which  it  would  appear  without  the  intervention 


OPTICS.  215 

of  any  optical  instrument  ;  and  this  magnifying  power  is 
greater  in  reflecting  than  in  refracting  telescopes. 

We  must  now  bring  our  observations  to  a  conclusion,  for  I 
have  communicated  to  you  the  whole  of  my  very  limited 
stock  of  knowledge  of  Natural  Philosophy.  If  it  will  enable 
you  to  make  further  progress  in  that  science,  my  wishes  will 
be  satisfied  ;  but  remember  that,  in  order  that  the  study  of 
nature  may  be  productive  of  happiness,  it  must  lead  to  an 
entire  confidence  in  the  wisdom  and  goodness  of  its  bounteous 
Author. 


FOR  THE 


EXAMI^ATIOJ^  OF  SCHOULRS. 


QUESTIONS  TO  CONVERSATION  L 


ON  THE  GENERAL  PROPERTIES  OF  BODIES. 

Introduction  ;  General  Properties  of  Bodies  ;  Impenetra- 
hility  ;  Extension  ;  Figure  ;  Divisibility  ;  Inertia  ; 
Attraction  ;  Attraction  of  Cohesion  ;  Density  ;  Rarity  ; 
Heat ;  Attraction  of  Gravitation* 


1.  What  is  to  be  understood  by  the  term  bodies,  as  used 
m  philosophy  ? 

2.  What  term  is  used  to  denote  substances  ? 

S.  What  properties  are  common  to  all  bodies  ? 

4.  Why  are  these  called  general  properties  of  bodies  r 

5.  What  is  impenetrability  ? 

6.  Can  liquids  occupy  the  same  space  of  a  solid  body  ? 

7.  How  can  you  prove  that   liquids  cannot  occupy  the 
same  space  occupied  by  solids  ? 

8.  Can  liquids  and  air  occupy  the  same  space  in  the  same 
time  ? 

9.  How  can  you  prove  that  they  cannot  ? 

10.  What  is  extension  ? 

11.  What  are  the  dimensions  of  a  body  ? 

12.  What  is  the  difference  between  height  and  depth,  as 
applied  to  extension  ? 

19 


218  GENERAL  PROPERTIES  OF  BODIES. 

13.  What  is  the  figure  of  a  body  ? 

14.  What  is  divisibility  in  natural  philosophy  ? 

15.  What  are  instances  of  practical  divisibility  of  matter' 
to  a  great  extent  ? 

16.  On  what  principle  is  it  that  we  can  smell  different 
odoriferous  objects  ? 

17.  If  we  inhale  particles  of  odoriferous  objects,  why  can 
we  not  see  these  particles  ? 

18.  If  the  particles  of  a  phial  of  fragrant  liquid  escape 
from  the  liquid  in  order  to  perfume  a  room,  does  the  liquid 
suffer  any  diminution  ;  and  if  so,  why  can  we  not  perceive  it, 
when  it  takes  place  ? 

19.  On  what  principle  are  wood  and  other  substances 
burnt,  as  it  is  termed,  when  commhted  to  the  fire  ? 

20.  Is  not  the  matter  of  which  wood  is  composed  destroy- 
ed, when  burnt  to  ashes  ;  and  if  not,  wliy  can  we  not  see  a 
greater  part  of  the  disunited  particles  ? 

21.  Is  it  then  a  principle  in  philosophy  that  there  has  been 
and  can  be  no  diminution  of  matter — not  of  a  single  particle  ? 

22.  What  is  inertia  ? 

23.  Will  bodies  always  remain  at  rest,  unless  an  external 
force  is  applied  to  them  ? 

24.  And  what  would  be  the  consequence  if  a  body  were 
put  in  motion  and  no  resistance  should  be  offered  ? 

25.  What  is  attraction? 

26.  Why  is  this  property  of  matter  called  the  attraction  of 
cohesion  ? 

27.  What  would  be  the  consequence  if  the  power  of  the 
attraction  of  cohesion  were  destroyed  ? 

28.  Does  the  attraction  of  cohesion  exist  also  in  liquids  ? 

29.  How  can  you  prove  that  it  exists  in  liquids  ? 

30.  Why  are  some  bodies  hard  and  others  soft  ? 

31.  To  what  is  the  cohesive  attraction  in  liquids  propor- 
tioned ? 

32.  Does  the  attraction  of  cohesion  exist  in  the  air  ? 

33.  But  are  the  particles  of  the  air  actually  under  the 
influence  of  this  attraction  ? 

34.  Why  are  they  not,  if  attraction  belongs  to  them  ? 

35.  How  do  we  know  that  attraction  does  belong  to  the 
air,  if  no  influence  is  exerted  upon  it  ? 

36.  Why  is  it  that  some  liquids  are  thick  and  others  thin  ? 

37.  What  is  density  ? 

38.  What  is  rarity  ? 


GENERAL  PROPERTIES  OF  BODIES.  219 

39.  How  are  we  to  judge  of  the  quantity  of  matter  in  bodies  ? 

40.  In  what  proportion  are  bodies  of  the  same  bulk  dense  ? 

41.  What  bodies  are  said  to  be  dense  ? 

42.  What  bodies  are  said  to  be  rare  ? 

43.  Why  are  not  sponge  and  cork  and  other  similar  sub- 
stances hard  since  their  particles  come  in  contact  ? 

44.  What  fluid  is  named  as  more  subtle  than  air  ? 

45.  What  effect  has  heat  on  bodies  ? 

46.  What  two  forces  are  said  to  act  always  on  bodies  in 
opposition  to  each  other  ? 

47.  In  what  cases  may  we  see  the  effect  of  heat  in  the 
expansion  of  bodies,  or  in  the  separation  of  their  particles  ? 

48.  How  are  liquids  made  to  boil  by  heat ;  or  how  is  the 
motion  or  agitation  of  boiling  liquids  produced  ? 

49.  Why  are  one^s  hands  and  fingers  swollen  or  larger 
on  being  held  near  thefire^  than  ivhen  exposed  to  the  cold? 

50.  Why  is  water  collected  in  drops  on  leaves  after  a  rain  ? 

51.  Does  rain  leave  the  clouds  in  the  form  of  drops,  as 
they  reach  the  earth  ? 

52.  How  then  do  these  drops  become  formed  ? 

53.  Whence  does  the  dew  collect  itself  into  drops  ? 

54.  What  causes  the  rain  or  water  to  fall  from  the  clouds  ? 

55.  What  causes  water  to  rise  in  a  capillary  tube,  above  its 
level  without  the  tube  ? 

56.  What  causes  water  to  rise  in  sponge  and  other  porous 
substances  above  its  level  ? 

57 »  If  several  tubes  of  different  bore  are  immersed  in  water^ 
in  which  will  it  rise  highest  ? 

58.  What  is  gravitation  ? 

59.  What  is  the  difference  between  cohesive  attraction  and 
gravitation  ? 

60.  What  causes  bodies  to  fall  to  the  earth  ? 

61.  In  what  proportion  do  bodies  attract  or  gravitate 
towards  each  other  ? 

62.  What  would  be  the  consequence  of  gravitation  on 
bodies,  were  it  not  for  cohesive  attraction  ? 

63.  W^hat  is  the  reason  that  cohesive  attraction  does  not 
operate  on  different  bodies  brought  into  contact  as  well  as  on 
the  particles  of  the  same  body  ? 

64.  When  will  the  surfaces  of  different  bodies  adhere  to 
nch  other  by  the  force  of  cohesive  attraction  ? 


QUESTIONS  TO  CONVERSATION  IL 


ON  THE  ATTRACTION  OF  GRAVITY. 

Attraction  of  Gravitation^  continued ;  Of  weight  ;  Of  the 
fall  of  Bodies  ;  Of  the  resistance  of  the  Air ;  Of  the 
Ascent  of  Light  Bodies, 


65.  AVhat  are  general  or  common  properties  of  bodies  ? 

66.  What  are  the  accidental  properties  of  bodies  ? 

67.  Are  color  and  weight  general  or  accidental  properties  ? 
6^.  What  is  weight,  or  of  what  is  it  the  effect  ? 

69.  If  bodies  mutually  attract  each  other,  why  is  not  the 
earth  drawn  to  other  bodies,  as  well  as  they  drawn  to  the 
earth  ? 

70.  If  there  were  but  one  body  in  the  universe,  would 
there  be  any  such  thing  as  weight  ? 

71.  Can  cohesive  attraction  exist  where  there  is  no  weight  ? 

72.  If  the  earth  attracts  all  objects  to  it,  why  are  not 
houses  and  other  objects  at  the  side  of  a  mountain  attracted 
or  drawn  away  from  their  foundations  ? 

7S.  Do  hills  and  mountains  possess  a  sideways  attraction  ? 

74.  How  can  it  be  proved  ? 

75.  Would  two  lines  suspended  by  weights  be  parallel  to 
each  other  ? 

76.  Wh}^  would  they  not  be  ? 

77'  If  they  are  not  parallel,  v/hy  do  we  not  perceive  their 
convergency  ? 

78.  What  is  the  object  of  the  Figure  1,  Plate  I.  ? 

79.  Do  heavy  and  light  bodies  fall  to  the  ground  with 
equal  rapidity  ? 

80.  Which  fall  with  the  greater  rapidity  ? 

81.  Why  do  heavy  bodies  fall  quicker  than  lighter  ones  ? 

82.  To  w4iat  is  the  resistance  of  the  air  to  falling  bodies 
proportioned  ? 

83.  Do  large  and  small  bodies  require  the  same  degree  of 
attraction  to  bnng  them  to  the  ground  in  the  same  time  ? 


ON  THE  ATTRACTION  OF  GRAVITY.  221 

84.  Which  require  the  greater  degree  of  attraction  ? 

85.  How  can  a  heavy  body  be  made  to  float  in  the  air 
instead  of  falling  immediately  to  the  ground  ? 

86.  Does  the  air  gravitate  towards  the  earth  ? 

87.  If  then  the  air  gravitates  towards  the  earth,  why  does 
it  not  fall  or  settle  completely  to  the  surface  of  the  earth  ? 

88.  What  two  forces  continually  operate  against  each 
other  on  the  air  ? 

89.  Is  the  air  of  the  same  density  at  the  surface  of  the  earth 
as  at  a  distance  from  it  ? 

90.  At  which  is  the  density  the  greatest  ? 

91.  Why  is  the  air  more  dense  at  the  surface  of  the  earth 
than  at  a  distance  from  it  ? 

92.  To  what  has  the  pressure  of  the  atmosphere  been 
compared  ? 

93.  What  bodies  do  not  gravitate  towards  the  earth  ? 

94.  How  does  gravity  operate  in  causing  smoke  and  steam 
to  ascend  ? 

95 ♦  How  high  will  smoke  and  steam  rise  before  they  re- 
main stationary  ? 

96.  Why  will  paper  rise  upon  the  top  of  water  instead  of 
sinking  to  the  bottom  like  a  stone  ? 

97«  On  what  principle  does  a  balloon  rise,  since  it  is  made 
of  materials  heavier  than  the  air  through  which  it  rises  ? 

98.  How  is  the  gravity  of  bodies  modified  by  the  eflect 
0^  the  air  ? 

99.  Can  a  feather  be  placed  in  a  situation  to  fall  as  quick  or 
as  heavy  as  stone  ? 

100.  How  can  it  be  done  ? 


19* 


QUESTIONS  TO  CONVERSATION  III. 


ON  THE  LAWS  OF  MOTION. 

On  Motion  ;  Of  the  Inertia  of  Bodies  ;  Of  Force  to  pro- 
duce Motion  ;  Direction  of  Motion  ;  Velocity ^  Absolute 
and  Relative  ;  Uniform  Motion ;  Retarded  Motion  ; 
Accelerated  Motion ;  Velocity  of  Falling  Bodies ; 
Momentum  ;  Action  and  Re-action  Equal  ;  Elasticity  of 
Bodies;  F  or  osity  of  Bodies  ;  Reflected  Motion  ;  Angle^i 
of  Incidence  and  Reflection, 


101.  On  what  is  the  science  of  mechanics  founded  ? 

102.  What  is  motion  ? 

103.  Can  a  body  move  itself  ? 

104.  What  is   the  power  called   which  puts  a  body   in 
motion  ? 

105.  What  is  it  that  binds  the  particles  of  a  body  together  ? 

106.  What  forces  them  asunder  ? 

107.  In  what  direction  is  the  motion  of  a  body  acted  on  by 
a  single  force  ? 

108.  What  is  the  velocity  of  motion  ? 

109.  To  what  is  velocity  proportioned  ? 

110.  What  is  absolute  velocity  ? 

111.  What  is  relative  velocity  ? 

112.  What  is  uniform  motion  ? 

113.  What  is  accelerated  motion  ? 

114.  What  is  retarded  motion  ? 

115.  What  are  instances  of  accelerated  motion  ? 

116.  What  are  instances  of  retarded  motion  ? 

117.  How  far  will  a  heavy  body,  suspended  in  air,  fall  the 
lirst  second  of  time  ? 

118.  How  far  the  second  ? 

119.  How  far  the  third  second  ? 

120.  How  does  the  time  of  an   ascending  body  thrown 
into  the  air,  always  compare  with  the  time  of  its  descent  ? 

121.  What  is  the  momentum  of  a  body  ? 


ON  THE  LAWS  OF  MOTION.  223 

122.  In  what  way  can  a  small  body  have  a  greater  mo 
mentum  than  a  large  one  ? 

123.  What  is  the  re-action  of  a  body  ? 

124.  To  what  is  the  re-action  of  a  body  equal  ? 

125.  What  is  the  object  of  Figure  3,  Plate  I.  ? 

126.  How  would  you  explain  that  figure  ? 

127.  How  would  you  explain  the  Figure  4,  in  Plate  I.  ? 

128.  Is  the  re-action  of  all  bodies  equal  to  the  action, 
when  a  blow  is  given  ? 

129.  In  what  bodies  is  it  equal  ? 

130.  In  w^hat  ones  is  it  not  equal  ? 

131.  What  is  the  object  of  Figure  5,  Plate  I.  ? 

132.  How  will  you  explain  it  ? 

133.  On  what  principle  is  it  that  birds  are  able  to  fly  ? 

134.  How  must  a  bird  strike  the  air  with  its  wings,  so  as  to 
remain  stationary  ? 

135.  How  so  as  to  rise  ? 

136.  How  so  as  to  descend  ? 

137.  Why  will  a  bird  remain  longer  in  the  air  with  its 
vv^ings  extended  than  when  they  are  closed j  although  they  are 
not  moved  ? 

138.  If  flying  is  only  the  effect  of  a  re-action,  why  could  not 
a  man  be  furnished  with  wings  so  as  to  fly  ? 

139.  How  is  swimming  effected  ? 

140.  On  what  principle  and  how  is  a  boat  moved  on  the 
water  ? 

141.  What  bodies  besides  air  are  elastic  ? 

142.  What  bodies  are  not  elastic  ? 

143.  What  is  it  that  produces  the  elasticity  of  bodies  ? 

144.  Is  it  supposed  that  ivory,  metals,  and  other  hard 
bodies  are  porous  ? 

145.  What  conjecture  did  Sir  Isaac  Newton  form  concer- 
ning the  porosity  of  the  earth  ? 

1 46.  What  is  reflected  motion  ? 

147.  If  a  ball  is  thrown  against  a  wall,  in  what  direction  is 
the  reflected  motion  ? 

148.  What  is  a  perpendicular  direction  ? 

149.  What  is  an  angle  ? 

150.  What  is  a  right  angle  ? 

151.  What  is  the  object  of  Figure  1,  Plate  II.  ? 

152.  How  are  all  circles  supposed  to  be  divided  ? 

153.  How  many  degrees  are  contained  in  the  two  angles 
formed  by  the  Figure  named  ? 


224  ON  COMPOUND  MOTIOX. 

154.  What  is  an  obtuse  angle  ? 

155.  How  would  you  explain  Figure  4,  Plate  II. : 

156.  What  is  an  angle  of  incidence  ? 
1 57-  What  is  an  angle  of  reflection  ? 

158.  How  does  the  angle  of  incidence  compare  with  the 
angle  of  reflection  as  to  size  ? 


QUESTIONS  TO  CONVERSATION  IV. 


ON  COMPOUND  MOTION. 

Compound  Motion^  the  result  of  two  Opposite  Forces ;  Of 
Circular  Motion,  the  Result  of  two  Forces  one  of  which 
confines  the  Body  to  a  Fixed  Point  ;  Centre  of  Motion, 
the  Point  at  Rest  while  the  other  parts  of  the  Body  move 
round  it  ;  Centre  of  Magnitude ,  the  Middle  of  a  Body  ;. 
Centripetcd  Force,  that  zvhich  confines  a  Body  to  a  fixed 
Central  Point  ;  Centrifugal  Force,  that  ivhich  impels  a 
Body  to  fly  from  the  Centre  ;  Fall  of  Bodies  in  a  Para- 
bola ;  Centre  of  Gravity,  the  Centre  of  Weight,  or  point 
about  which  the  Parts  balance  each  other. 


150.  What  is  compound  motion  ? 

160.  How  would  a  body  move  struck  by  two  equal  forces 
in  opposite  directions  ? 

161.  What  is  the  object  of  Figure  5,  Plate  n.  ? 

162.  How  would  you  explain  that  figure? 

163.  What  is  the  design  of  Figure  6,  Plate  II.  ? 

164.  How  would  you  explain  it  ? 

165.  What  is  the  line  A.  D.  called  in  the  Fisfure  5  of  Plate 
II.? 

166.  What  is  the  line  A.  D.  called  in  Figure  6,  Plate  11.  ? 

167.  What  are  the  lines  A.  D.  and  B.  C.  called  in  Figure 
7,  Plate  II.  ? 

168.  Of  what  is  circular  motion  the  result  ? 

169.  What  instance  of  circular  motion  thus  produced  could 
you  give  ? 


ON  COMPOUND  MOTION.  225 

170.  What  is  the  axis  of  motion  ? 

171.  Is  the  velocity  of  motion  the  same  at  a  distance  from 
as  near  the  centre  of  motion  ? 

172.  What  is  the  object  of  Figure  1,  Plate  III.  ? 

173.  What  are  the  forces  called  in  circular  motion^  that 
balance  or  act  in  opposition  to  each  other  ? 

174.  What  is  centripetal  motion  ? 

175.  What  is  centrifugal  motion  ? 

176.  What  would  be  the  consequence  if  in  circular  motion 
the  centripetal  should  be  destroyed  ? 

177.  What  is  a  tangent  ? 

178.  How  would  you  describe  Figure  2  in  Plate  III.  ? 

179.  If  curved  lines  or  circular  motion  are  produced  by 
the  operation  of  two  forces,  how  is  it,  that  the  falling  of  a 
stone  thrown  into  the  air  is  in  a  circular  direction  ? 

180.  What  is  a  parabola  ? 

181.  Why  will  a  stone  thrown  perpendicularly  into  the  air 
descend  perpendicularly  ? 

182.  What  is  the  centre  of  gravity  ? 

183.  What  part  of  a  body  must  be  supported  to  keep  the 
body  from  falling. 

184.  What  would  be  the  consequence  if  the  centre  of 
gravity  were  not  supported  ? 

185.  What  causes  or  when  will  a  loaded  carriage  be  turned 
over  or  upset  ? 

186.  How  would  you  explain  Figure  4  in  Plate  III.  ? 
187-  How  is  it  that  rope  dancers  are  able  to  perform  their 

feats  of  agility  without  falling  ? 

188.  W'hy  do  persons  in  ascending^  a  hill  incline  or  stoop 
forward,  and  in  descending  a  hill  incline  backwards  ? 

189.  How  would  you  explain  Figure  5  in  Plate  III.  ? 

190.  Is  the  centre  of  gravity  always  in  the  middle  of  a  body  ? 

191.  When  is  the  centre  of  magnitude  the  centre  of  gravity  ? 

192.  What  bodies  are  the  most  easily  upset  ? 

193.  Why  can  a  person  carry  two  pails  of  water,  one  in 
each  hand,  easier  than  one  pail  ? 

194.  How  would  you  explain  Figure  6,  Plate  III.  ? 

195.  Where  must  the  line  falling  perpendicularly  from  the 
centre  of  gravity  in  a  wine  glass  strike  the  table  on  which  it 
stands  so  as  not  to  be  turned  over  ? 

196.  How  are  two  bodies  connected  together  to  be  con- 
sidered as  to  their  centre  of  gravity  ? 

1 97-  How  are  Figui^s  7, 8  and  9  in  Plate  III.  to  be  explained  ? 


QUESTIONS  TO  CONVERSATION  V. 


OjY  the  mechanical  powers. 

Of  the  Foicer  of  Machines  ;  Of  the  Lever  in  General  ;  Of 
the  Lever  of  the  First  Kind.,  having  the  Fulcrum  between 
the  Power  and  the  Weight ;  Of  the  Lever  of  the  Second 
Kind,  having  the  Weight  between  the  Power  and  the 
Fulcrum;  Of  the  Lever  of  the  Third  Kind y  having  the 
Power  betiveen  the  Fulcrum  and  the  Weight. 


198.  How  many  of  the  mechanical  powers  are  there  ? 

199.  What  are  they  called  ? 

200.  In  order  to  understand  the  power  of  a. machine,  how 
many  things  are  to  be  considered  ? 

201.  What  is  the  first  .^ 

202.  What  is  the  second  ? 

203.  What  is  the  third  ? 

204.  What  is  the  fourth  ? 

205.  What  is  the  lever  in  mechanics  ? 

206.  What  is  the  fulcrum  in  a  pair  of  scales  ? 

207.  Why  are  the  scales  in  F ig.  1 ,  Plate  IV.  in  equilibrium  ? 

208.  What  is  the  centre  of  gravity  to  two  scales  in  equili- 
brium ? 

209.  In  what  way  can  a  level  be  used  so  that  bodies  of 
different  weights  may  balance  each  other  ? 

210.  What  are  the  arms  of  a  lever  ? 

211.  Why  are  not  the  two  arms  of  a  lever  in  equilibrium, 
though  of  unequal  length  ? 

212.  How  can  the  two  arms  of  a  lever  be  brought  intQ, 
equilibrium  ? 

213.  What  is  the  design  of  Figure  4^  Plate  IV.  ? 

214.  How  many  kinds  of  levers  are  there  ? 

215.  Where  is  the  fulcrum  in  the  first  kind  ? 

216.  How  are  we  to  use  the  lever  in  raising  or  lifting  large 
weights  ? 

217.  What  power  of  mechanics  do  scissors  involve  ? 


ON  THE  MECHANICAL  POWERS,  22^ 

218.  How  may  the  scissors  be  explained  as  formed  by  the 
lever  ? 

219.  How  is  the  second  kind  of  levers  designed  ? 

220.  What  are  the  most  common  examples  of  levers  of 
the  second  kind,  or  where  the  weight  is  placed  between  the 
fulcrum  and  the  power  ? 

221.  How  would  you  explain  the  opening  of  a  common 
door,  as  involving  the  principle  of  the  second  kind  of  levers  ? 

222.  What  is  the  third  kind  of  levers  ? 

223.  What  is  an  instance  of  its  use  ? 

224.  How  does  the  raising  a  weight  by  the  hand  represent 
this  kind  of  levers  ? 


QUESTIONS  TO  CONVERSATION  V, 
CONTINUED. 

ON  THE  MECHANICAL  POWERS. 

Of  the  Pulley  ;  Of  the  Wheel  and  Axle  ;   Of  the  Inclined 
Plane  ;  Of  the  Wedge  ;  Of  the  Screw, 


k 


225.  What  is  the  second  mechanical  power  ? 

226.  What  is  a  pulley  ? 

227.  What  is  the  fulcrum  of  a  pulley  ? 

228.  What  is  the  design  of  Figure  1,  Plate  V.  ? 

229.  How  would  you  explain  Figure  2,  in  Plate  V.  ? 

230.  In  what  does  the  advantage  of  a  moveable  ^nWey 
consist  ? 

231.  How  do  the  weight  and  power  of  a  moveable  pulley 
compare,  that  their  momenta  be  equal  ? 

232.  In  the  use  of  the  moveable  pulley  is  there  no  loss  of 
time  ? 

233.  And  to  what  is  the  loss  of  time  proportioned  ? 

234.  What  then  is  the  advantage  of  this  pulley,  if  there  is 
as  much  loss  in  time  as  gain  in  power  ? 

235.  If  there  is  no  gain  in  time  or  power  from  the  use  of 
the  fixed  pulley,  why  is  it  used  ?  ^ 


228  ON  THE  MECHANICAL  POWERS. 

236.  What  is  the  third  mechanical  power  .^ 

237.  What  is  the  design  of  Figure  5,  Plate  V.  ? 

238.  In    what   proportion   is    the   power   of   the   whe^l 
increased  .^ 

239.  How  may  a  wheel  be  compared  to  the  lever  .^ 

240.  How  would  you  explain  Figure  6  in  Plate.  V.  ? 

241.  On  what  mechanical  force  is  the  windmill  operated  ? 

242.  What  is  the  fourth  mechanical  power  ? 

243.  What  is  an  incHned  plane  ? 

244.  How  would  you  explain  Figure  6,  Plate  V.  ? 

245.  What  is  the  fifth  mechanical  power? 

246.  Of  what  is  the  wedge  composed  ?  . 

247.  In  what  does  the  resistance  to  the  wedge  consist  ? 
24S .  On  w^hat  mechanical  principle  are  cutting  instruments 

designed  ? 

249.  What  is  the  last  mechanical  power  ? 

250.  To  which  of  the  other  mechanical  powers  is  the 
screw  referable  ? 

251.  What  diminishes  the  force  of  all  machinery  ? 

252.  What  do  we  understand  by  friction  in  machinery  ? 

253.  In   what    proportion   is   the  friction  of  machinery 
di  ninished  ? 

254.  What  is  the  reason  for  putting  oil  or  grease  upon  the 
axles  of  wl  ee  s  and  on  other  machinery  ? 

255.  H«  w  many  kinds  of  friction  are  there  ? 

256.  What  are  they  .^ 

257.  Which  will  the  most  readily  overcome  obstacles,  a 
large  or  small  wheel  ? 

258.  Why  is  a  wheel  fastened  on  descending  a  hill  ? 

259.  What  besides  friction  diminishes  the  force  of  ma- 
chinery ? 

260.  In  what  state  would  the  force  of  machinery  be  perfect-^ 


QUESTIONS  TO  CONVERSATION  VL 


CAUSES  OF  THE  EARTH'S  ANNUAL  MOTION. 

Of  the  Planets,  and  tlieii^  Motion  ;   Of  the  Diurnal  Motion 
of  the  Earth  and  Planets, 


261.  If  bodies  attract  each  other  in  proportion  to  the 
quantity  of  matter,  why  does  not  the  sun  attract  the  earth 
completely  to  itself.^ 

262.  If  the  earth  on  its  creation  had  been  put  in  motion  by 
a  single  force  without  any  resistances,  what  would  have  been 
its  course  ? 

263.  How  would  you  explain  Figure  1,  Plate  VI.  ? 

264.  In  that  figure  in  what  direction  does  the  attraction  of 
the  sun  operate  on  the  projectile  force  of  the  earth  ? 

265.  When  two  forces  operate  perpendicularly  on  each 
other,  in  what  direction  will  be  their  compound  motion  ? 

266.  Why  then  is  the  line  A.  D.  in  Figure  1,  circular 
instead  of  a  right  line  diagonal  to  the  parallelogram,  A  B 

267.  How  would  you  explain  that  part  of  Figure  1 ,  mark- 
ed D  F  G  E  ? 

268.  And  that  part  marked  G  H  I  K  ? 

269.  What  is  the  centripetal  force  of  the  earth  ? 

270.  What  is  its  centrifugal  force  ? 

271.  How  would  you  explain  Figure  2,  Plate  VI.  ? 

272.  In  Figure  3,  Plate  VI,  why  is  the  earth  carried  in 
the  line  A  B  instead  of  the  line  A  C  according  to  the  princi- 
ple of  Figure  1  ? 

273.  When  the  earth  arrives  at  E,  why  does  it  not  move 
round  the  sun  in  a  small  orbit  instead  of  receding  off  from  the 
sun  as  at  G  ? 

274.  What  is  the  object  of  Figure  4,  Plate  VI.  ? 

275.  What  is  that  part  of  the  earth's  orbit  called  farthest 
from  the  sun  ?      ^ 

276.  What  is  that  nearest  the  sun  ? 

20 


230  ON  THE  PLANETS. 

277.  How  much  nearer  is  the  earth  to  the  sun  in  its  parhe- 
lion than  at  its  aphelion. 

278.  Is  the  earth  nearest  the  sun  in  summer  or  winter  ? 

279.  How  can  we  account  for  its  being  coldest  when  near- 
est the  sun  ? 

280.  How  much  longer  is  the  earth  performing  the  sum« 
mer  half  than  the  winter  half  of  its  orbit  ? 

281.  What  are  the  planets  ? 

282.  If  they  are  worlds  like  our  own,  why  do  they  appear 
so  small  ? 

283.  If  the  fixed  stars  are  suns,  with  planets  revolving 
round  them,  why  should  we  not  see  those  planets  as  well  a§ 
their  suns  ? 

284.  Why  do  we  not  see  the  stars  in  the  day  time  ? 

285.  If  the  planets  are  only  seen  by  the  reflected  light  of 
the  sun,  how  is  it  that  they  can  be  seen  in  the  night  ? 

286.  What  motion  have  the  planets  besides  that  about  the 
sun.^ 


QUESTIONS  TO  CONVERSATION  VH. 


ON  THE  PLANETS. 

Of  the  Satellites  or  Moons  ;  Gravity  diminishes  as  the 
Square  of  the  distance  ;  Of  the  Solar  System ;  Of 
Comets ;  Constellations^  Signs  of  the  Zodiac ;  Of 
Copernicus^  Newton^  Sfc, 


287.  How  are  the  planets  distinguished  ? 

288.  Which  are  the  primary  planets  ? 

289.  Which  are  the  secondary  ? 

290.  By  what  names  are  the  secondary  planets  called  ? 

291.  Why  does  not  the  sun  attract  the  secondary  planets 
from  their  primaries  ? 

292.  To  what  is  the  force  of  attraction  proportioned  besides 
the  quantity  of  matter  in  the  attracting  bodies  ? 


ON  THE  PLANETS.  23/ 

293.  In  what  proportion  is  force  of  attraction  diminished 
by  distance  ? 

294.  What  motion  has  the  earth  besides  that  about  the  sun 
and  on  its  own  axis  ? 

295.  Do  the  planets  revolve  round  the  centre  of  the  sun  ? 

296.  Around  what  point  do  they  revolve  .^ 

297.  Has  the  sun  any  motion  f 

298.  How  is  it  known,  that  the  sun  turns  on  its  own  axis  ? 

299.  What  motion  has  the  sun  besides  that  around  its  axis  ? 

300.  Do  the  planets  at  different  distances  revolve  round 
the  sun  in  the  same  time  ? 

301.  Why  do  the  more  distant  from  the  sun  revolve  slower 
than  those  nearer  to  it  ? 

302.  What  is  the  object  of  Figure  1,  Plate  VII.  ? 

303.  What  is  the  object  of  Figure  2  of  that  plate  ? 

304.  How  far  from  the  sun  is  Mercury  ? 

305.  In  what  time  does  it  revolve  round  the  sun  ? 

306.  How  far  is  Venus  from  the  sun  ? 

307.  In  what  time  does  it  revolve  round  the  sun  ? 

308.  By  what  names  do  we  call  Venus  .^ 

309.  How  far  is  the  earth  from  the  sun  ? 

310.  In  what  time  does  it  revolve  round  the  sun  7 

311.  Which  planet  is  next  to  the  earth  in  distance  from 
the  sun  ? 

312.  How  far  is  Mars  from  the  sun  ? 

313.  How  long  time  is  occupied  in  his  revolution  about 
the  sun  ? 

314.  What  four  small  planets  are  next  to  Mars  in  distance 
from  the  sun  ? 

315.  AVhat  did  Dr.  Herschel  call  these  planets? 

316.  What  is  the  distance  of  Jupiter  from  the  sun 

317.  In  what  time  is  its  revolution  performed  } 

318.  How  much  larger  is  Jupiter  than  our  earth  ? 

319.  How  many  moons  or  satellites  has  Jupiter  ? 

320.  Which  planet  is  next  in  order  ? 

321.  How  far  is  Saturn  from  the  sun  ? 

322.  In  what  time  does  it  revolve  round  the  sun  ? 

323.  How  many  moons  has  Saturn  ? 

324.  How  is  Saturn  distinguished  from  the  other  planets  ? 

325.  How  many  moons  has  Herschel  or  the  Georgium 
Sidus  ? 

326.  How  much  more  light  and  heat  have  we  than  Sa- 
turn ? 


232  ON  THE  PLANETS. 

327.  Are  comets  supposed  to  be  planets  ? 

328.  What  are  the  constellations  ? 

329.  What  are  their  names  ? 

330.  How  would  you  explain  Figure  1,  Plate  VIII.  ? 

331.  What  is  to  be  understood  by  the  signs  or  constella- 
tions  being  in  the  zodiac  ? 

332.  On  what  is  the  different  size  and  brilliancy  of  the 
fixed  stars  depending  ? 

333.  How  may  a  fixed  star  he  easily  distinguished  from 
a  planet  ? 

334.  By  ivhat  is  the  twinkling  light  of  the  fixed  stars 
occasioned  ? 

335.  If  the  earth  is  continually  revolving  on  its  axis,  wh}/ 
do  we  not  perceive  its  motion  ? 

336.  In  case  the  earth  thus  revolves  every  24  hours,  do 
not  the  Sun  and  stars  appear  to  us  as  if  they  revolved  about 
the  earth  ? 

337.  How  would  you  illustrate  this  ? 

338.  Why  is  it  more  probable  the  earth  revolves  than  that 
the  Sun  and  stars  do  ? 

339.  How  fast  would  a  person  move  in  the  latitude  of 
London  by  the  motion  of  the  earth  upon  its  axis  ? 

340.  How  fast  does  the  earth  move  about  the  Sun  ? 

341.  What  was  the  system  of  Ptolemy  concerning  as- 
tronomy ? 

342.  What  is  the  present  system  of  astronomy  called  ? 

343.  Who  was  the  founder  of  the  present  system  of  as- 
tronomy ? 

344.  When  was  the  Copernican  system  of  astronomy 
adopted  ? 

345.  What  important  discovery  did  Newton  make  in  regard 
io  the  Copernican  system  ? 

346.  What  led  Newton  to  his  discoveries  ? 

347.  Hjw  much  greater  is  the  diameter  of  the  sun  than  of 
the  earth  ? 

348.  HoiD  much  does  its  cubit  magnitude  exceed  that  of  the 
earth  ? 

349.  Hitc  is  it  known  that  the  sun  revolves  upon  its  axis  ? 

350.  What  does  Dr,  Herschel  suppose  the  dark  spots  on 
the  sun^s  disk  to  be  ? 


QUESTIONS  TO  CONVERSATION  VIIL 


ON  THE  EARTH. 

On  the  Terrestrial  Globe  ;  Of  the  Figure  of  the  Earth  ;  Of 
the  Pendulum  ;  Of  the  Variation  of  the  Seasons,  and  of 
the  Length  of  Days  and  Nights  ;  Of  the  causes  of  the 
Heat  of  Summer  ;  Of  Solar,  Siderialy  and  Equal  or 
Mean  Time. 


351.  How  does  it  appear  that  the  earth  is  of  a  globular 
form  ? 

352.  What  is  the  axis  of  the  earth  ? 

353.  What  are  the  poles  ? 

354.  What  is  the  equator  ? 

355.  What  is  the  ecliptic  ? 

356.  What  is  the  earth's  orbit? 

357'  If  the  ecliptic  relates  only  to  the  heavens,  why  is  it 
described  on  the  terrestrial  globe  ? 

358.  What  does  Figure  1,  Plate  IX.  represent  ? 

359.  What  are  the  zones  ? 

360.  What  is  the  torrid  zone  ? 

361.  Where  are  the  temperate  zones  ? 

362.  Where  are  the  frigid  zones  ? 

363.  What  are  the  meridian  lines  ? 

364.  When  will  it  be  twelve  o'clock  at  noon  to  all  places 
under  any  particular  meridian  ? 

365.  What  circles  are  called  greater  circles  ? 

366.  What  ones  are  called  lesser  circles  ? 

367.  Into  how  many  parts  are  circles  divided  ? 

3 68.  What  is  the  diameter  of  a  circle  ? 

369.  How  many  degrees  does  the  diameter  of  a  circle 
contain  ? 

370.  How  many  degrees  are  there  in  a  meridian  reaching 
from  one  pole  to  the  other  ? 

371.  How  many  degrees  are  there  between  the  equator 
and  the  poles  ? 

20* 


234  ON  THE  EARTH. 

372.  Are  the  degrees  of  longitude  in  different  latitudes  ol 
the  same  length  ? 

37s.  What  is  the  length  of  a  degree  of  latitude  ? 

374.  What  is  the  reason  that  a  degree  of  longitude  on  the 
equator  is  not  the  same  as  a  degree  of  latitude  ? 

37 o.  Why  is  the  earth  supposed  to  be  protuberant  at  the 
equator  ? 

376,  Why  does  the  head  of  a  person  in  the  motion  occa- 
sioned by  the  revolution  of  the  earth  on  its  axis  move  faster 
than  his  feet  ? 

377'  W^hat  is  a  sphere  or  globe  ? 

378.  Will  the  same  body  weigh  the  same  at  the  equator  as 
at  the  poles  ? 

379-  At  which  is  it  the  heaviest  ? 

380.  Why  is  it  heaviest  at  the  poles  ? 

381.  Were  one  to  penetrate  deep  into  the  earth,  would 
the  force  of  gravity  increase  ? 

382.  Why  not  ? 

383.  Has  an  attempt  ever  been  made  to  ascertain  whether 
bodies  will  weigh  heavier  at  the  poles  than  at  the  equator  ? 

384.  By  whom   ? 

385.  Could  the  experiment  be  made  by  the  common 
scales  ? 

386.  Why  not  ? 

387-  By  what  was  it  made  ? 

388.  Why  does  a  pendulum  vibrate  ? 

389.  Why  are  not  its  vibrations  perpetual  ? 

390.  How  can  a  pendulum  determine  whether  objects  are 
heavier  at  the  poles  or  at  the  equator  ? 

391.  Why  will  a  pendulum  vibrate  slower  by  increasing 
its  length  ? 

392.  Hoiv  do  the  pendulums  used  at  the  equator  and  at 
•polar  regions  compare  in  length  ? 

393.  IVhy  does  a  clock  go  faster  in  lointer  than  in 
summer  ? 

394.  What  causes  the  variation  of  seasons^  and  the  change 
of  day  and  night  ? 

395.  How  much  is  the  axis  of  the  earth  inclined  to  the 
plane  of  the  ecliptic  ? 

396.  What  are  nodes  ? 

397.  How  would  you  explain  Figure  2  in  Plate  IX.  ? 

398.  Why  in  the  polar  regions  is  it  six  months  day  and  six 
months  night  ? 


ON  THE  EARTH*  235 

399.  Why  are  the  points  where  the  ecliptic  cuts  the  equa- 
tor called  the  equinoxes  ? 

400.  What  point  in  ecliptic  is  called  the  summer  solstice  ? 

401.  What  is  the  winter  solstice  ? 

402.  Why  is  the  heat  greater  at  the  equator  than  at  the 
distance  from  it  ? 

403.  Why  is  the  heat  of  perpendicular  rays  more  intense 
than  that  of  oblique  ones  ? 

404.  By  which  figure  is  this  illustrated  ? 

405.  What  is  the  object  of  figure  3,  Plate  X.  ? 

406.  What  is  the  object  of  Figure  4,  Plate  X.  .^ 

407.  Wiiy  does  the  Sun  give  more  heat  at  mid-day,  than 
in  the  morning  and  towards  evening  ? 

408.  Why  is  it  warmer  in  July  and  August  than  in  June, 
when  the  days  are  the  longest ;  and  at  2  and  3  P.  M.  than 
at  noon  ? 

409.  Have  the  other  planets  the  same  vicissitudes  of 
seasons  ? 

410.  How  is  it  that  the  earth  performs  366  complete  rev- 
olutions in  a  3^ear  which  has  but  36j  days  and  nights  ? 

411.  Why  do  the  fixed  stars  appear  to  go  round  the  earth 
quicker  than  the  sun  ? 

412.  What  is  a  siderial  day  ? 

413.  What  is  a  siderial  year  ? 

414.  What  is  a  solar  day  ? 

415.  What  is  a  solar  year  ? 

416.  How  can  we  know  when  the  earth  has  performed 
one  complete  revolution  about  the  Sun  ? 

417.  How  would  you  explain  Figure  1,  Plate  XI.? 


QUESTIONS  TO  CONVERSATION  IX. 


ON  THE  MOON. 


Of  the  Moon^s  Motion  ;  Phases  of  the  Moon  ;  Eclipses  of 
the  Moon  ;  Eclipses  of  Jupiter^ s  Moons  ;  Of  the  Latitude 
and  Longitude  ;  Of  the  Transits  of  the  Inferior  Planets  ; 
Of  the  tides. 


418.  In  what  time  does  the  moon  revolve  about  the  earth  ? 

419.  In  what  time  does  the  moon  revolve  on  its  axis  ? 

420.  How  is  it  known  how  long  it  takes  the  moon  to 
revolve  on  its  axis  ? 

421.  Does  the  earth  exhibit  the  same  changes  to  the  moon, 
that  the  moon  exhibits  to  the  earth  ? 

422.  What  are  these  changes  called  ? 

423.  How  would  you  explain  Figure  2,  Plate  XI.  ? 

424.  When  is  the  moon  said  to  be  in  conjunction  with  the 
sun  ? 

425.  How  is  the  moon  eclipsed.^ 

426.  How  is  the  sun  eclipsed  ? 

427.  At  what  time  of  the  moon  can  she  be  eclipsed  ? 

428.  At  what  time  of  the  moon  can  the  sun  be  eclipsed  ? 

429.  As  the  moon  passes  between  the  sun  and  the  earthy 
and  as  the  earth  passes  between  the  sun  and  the  moon,  once 
every  month,  why  do  we  not  have  a  lunar  and  solar  eclipse 
every  month  ? 

430.  When  does  a  partial  eclipse  take  place  ? 

431.  What  is  the  object  of  Figure  1,  Plate  XH.  ? 

432.  How  can  the  comparative  size  of  the  earth  and  moon 
be  determined  by  a  lunar  eclipse  ? 

433.  How  would  you  explain  Figure  2,  Plate  XII.  ? 

434.  When  is  the  earth  eclipsed  to  the  moon  ? 

435.  Are  the  eclipses  of  the  distant  planets  frequent  ? 

436.  What  benefit  do  we  derive  from  them  ? 

437»  How  can  the  latitude  of  a  place  be  determined  ? 


ON  THE  MOON.  237 

438.  How  can  the  longitude  of  a  place  be  determined  by 
observation  of  the  heavenly  bodies  ? 

439.  How  can  it  be  determined  at  sea  by  the  use  of  two 
watches  ? 

440.  What  occasions  the  tides  ? 

441.  Why  are  the  tides  occasioned  by  the  Moon^  unless 
water  is  acted  on  more  powerfully  by  gravitation  than  the 
land  ? 

442.  If  the  tides  are  occasioned  by  the  attraction  of  the 
moon,  lohy  is  there  a  high  tide  on  the  side  of  the  earth 
opposite  the  moon  as  ivell  as  on  that  next  to  it  ? 

443 »  How  will  you  explain  Figure  3,  Plate  XH.  ? 

444.  As  the  attraction  of  the  Sun  is  greater  than  that  of 
the  moon,  why  does  not  the  sun  produce  the  chief  influence 
in  the  tides  ? 

445.  But  does  the  Sun  exercise  no  influence  in  the  produc- 
tion of  the  tides  ? 

446.  When  does  the  sun  increase  the  tides  ? 

447.  What  is  meant  by  the  sun  and  moon  acting  in 
conjunction  on  the  tides  ? 

448.  What  are  tides  called  produced  by  the  conjunctive 
attraction  of  the  sun  and  moon  ? 

449.  What  are  neap  tides  ? 

450.  How  would  you  explain  Figure  4,  Plate  XII.  ? 

451.  How  would  you  explain  Figure  5,  Plate  XH.  ? 

452.  In  what  parts  of  the  earth  are  tides  the  highest  ? 

453.  Why  are  they  highest  at  the  equator  ? 

454.  Why  are  the  tides  three  quarters  of  an  hour  later 
5? very  day  ? 

455.  Why  are  tides  highest  at  the  new  and  full  of  the  moon  ? 


QUESTIONS  TO  CONVERSATION  X. 


ON  THE  MECHANICAL  PROPERTIES  OF  FLUIDS, 

Dejinition  of  a  Fluid ;  Distinction  between  Fluids  and 
Liquids  ;  Of  Non-Elastic  Fluids,  Scarcely  Susceptible 
of  Compression  ;  Of  the  Cohesion  of  Fluids  ;  Of  their 
Gravitation  ;  Of  their  Equilibrium  ;  Of  their  Pressure  ; 
Of  Specific  Gravity  ;  Of  the  Specific  Gravity  oj  Bodies 
Heavier  than  Water  ;  Of  those  of  the  Same  Weight  as 
Water  ;  Of  those  Lighter  than  Water  ;  Of  the  Specific 
Gravity  of  Fluids* 


456.  What  is  the  science  called,  that  treats  of  the  mechan- 
ical properties  of  fluids  ? 

457.  What  is  a  fluid  ? 

458.  In  which  is  the  attraction  of  cohesion  the  most  pow- 
erful, solids  or  fluids  ? 

459.  What  is  the  distinction  between  a  liquid  and  a  fluid  .^ 

460.  Are  water  and  other  liquids  compressible  ? 

461.  Why  are  they  not  ? 

462.  What  reason  is  therefor  supposing  that  the  parti- 
cles of  fluids  are  round  ? 

463.  What  experiment  has  been  made  to  shew  that  liquids 
are  not  compressible  ^^ 

464.  Why  cannot  liquids  be  moulded  into  figures,  like 
solids  ? 

465.  What  is  meant  by  the  level  or  equihbrium  of  fluids  ? 

466.  Why  will  oil  remain  upon  the  top  of  water  ? 

467.  Why  is  the  resistance  of  fluids  less  than  that  of  solids  ? 

468.  Why  are  fluids  inclined  to  a  state  of  equilibrium  ? 

469.  Why  will  water  and   other  liquids  run  out  of  any 
opening  to  the  vessel  containing  them  ? 

470.  From  what  does  the  lateral  pressure  of  liquids  pro-   k 
ceed  ?  I 

471.  To  what  is  the  velocity  proportioned  of  liquids  issu- 
ing from  an  orifice  in  the  side  of  a  vessel  containing  them  ? 


OP  SPRINGS,  FOUNTAINS;  &C.  235 

472.  How  would  you  explain  figure  5,  Plate  XIII ,  ? 

473.  From  what  proceeds  the  upward  pressure  of  liquids  ? 

474.  How  would  you  explain  figure  4,  Plate  XIII.  ? 

475.  What  is  the  object  of  figure  6,  Plate  XIII.  ? 

476.  What  is  the  specific  gravity  of  a  body  ? 

477.  To  what  is  the   specific  gravity  of  bodies  propor- 
tioned ? 

478.  What  standard  has  been  fixed  on  to  determine  the 
specific  gravity  of  different  bodies  ? 

479.  Why  will  a  body  weigh  less  in  the  water  than  out 
of  it? 

480.  To  what  is  the  resistance  of  water  to  a  body  im- 
mersed in  it  proportioned  ? 

481.  How  much  does  a  body  weighed  in  the  water  lose  of 
its  weight  ? 

482.  What  is  the  specific  gravity  of  gold  ? 

483.  What  solid  is  there  of  the  same  specific  gravity  of 
water  ? 

484.  How  will  a  body  of  the  specific  gravity  of  water  re- 
main in  water  ? 

485.  How  is  the  specific  gravity  of  fluids  ascertained  ? 

486.  How  would  you  explain  figure  8,  Plate  XIII.  ? 


QUESTIONS  TO  CONVERSATION  XI. 


OF  SPRINGS,  FOUNTAINS,  &c. 

Of  the  Ascent  of  Vapor  and  the  Formation  of  Clouds  ;  Of 
the  Formation  and  Fall  of  Rain^  ^c.  ;  Of  the  Forma-" 
Hon  of  Springs  ;  Of  Rivers  and  Lakes  ;  Of  Fountains. 


487.  What  is  the  reason  that  the  great  quantity  of  rain 
which  falls  upon  the  earth  and  sinks  into  it,  does  not,  in  the 
course  of  time,  injure  its  solidity  ? 

488.  How  are  clouds  formed  ? 

489.  But  since  the  watery  vapor  is  lighter  than  the  air, 
why  does  it  not  continue  to  rise ;  and  why  does  it  unite  again 
to  form  clouds  ? 


240  MECHANICAL  PROPERTIJES  OP  AIR. 

490.  What  prevents  the  clouds  remaining  in  the  atmos^ 
phere,  where  they  are  formed  ? 

491.  Why  do  the  clouds  descend  to  the  earth  in  drops  of 
water  instead  of  vapor,  as  they  ascend  ? 

492.  What  is  the  difference  between  rain  and  spring 
water  ? 

493.  Which  is  the  most  pure  ? 

494.  How  are  rivulets,  at  first,  formed  ? 

495.  How  would  you  explain  figure  9>  Plate  XIII.  ? 

496.  How  high  may  a  spring  rise  ? 

497.  On  what  principle  does  water  ascend  as  well  as  de- 
scend in  its  course,  as  is  often  the  case,  in  carrying  it  by  the 
use  of  ducts  ? 

498.  How  would  you  explain  figure  1,  Plate  XIV.  ? 

499.  What  is  the  name  of  the  cup  made  on  the  principle 
of  this  figure  ? 

500.  Why  must  wells  on  high  land,  as  on  hills,  be  dug 
deep  in  order  to  be  supplied  with  water  r 

501.  Why  do  rivers  generally  have  their  source  in  moun- 
tainous regions  ? 

502.  How  was  the  lake  Geneva  probably  formed  ? 

503.  How  would  you  explain  figure  2,  Plate  XIV.  ? 


QUESTIONS  TO  CONVERSATION  XH. 


ON  THE  MECHANICAL  PROPERTIES  OF  AIR. 

Of  the  Spring  or  Elasticity  of  the  Air  ;   Of  the  weight  of 
the  Air  ;  Experiments  ivith  the  Air  Pump  ;   Of  the  Ba-  » 
rometer  ;  Mode  of  Weighing  Air  ;  Specific  Gravity  of\ 
Air ;   Of  Pumps  ;  Description  of  the  Sucking  Pump  ; 
Description  of  the  Forcing  Pump. 


504.  How  are  the  fluids   called  air    distinguished  from 
liquids  ? 

505.  What  effect  does  heat  have  on  elastic  fluids  ?  , 

506.  To  what  distance  from  the  earth  does  the  atmosphere^) 
extend  ? 


MECHANICAL  PROPERTIES  OF  AIR.  241 

507.  What   weight  of  air  is  a  common  or  middling  size 
man  supposed  to  sustain  ? 

508.  Why  does  not  such  a  weight  crush  him  to  atoms  ? 

509.  What  would  be   the   consequence  if  the   weight  of 
external  air  were  removed  from  us  ? 

510.  Why  do  not  bodies  of  various  weights  in  the  open 
air  fall  in  the  same  time  ? 

511.  How  may  it  be  shown  that  the  air  has  weight  ? 

512.  How  may  the  power  of  expansion  in  air  be  ascer* 
tained  ? 

513.  How  much  does  a  column  of  air  reaching  to  the  top 
of  the  atmosphere  of  an  inch  in  diameter,  weigh  ? 

514.  How  can  the  weight  of  a  small  quantity  of  air  be 
ascertained  ? 

515.  How  much  heavier  Is  water  than  air  ? 

516.  How  can  the  weight  of  the  atmosphere  be  determined 
by  a  Barometer  ? 

517.  When  is  the  air  the  heaviest,  in  wet  or  in  dry  weather? 

518.  But   why  do  our  feelings  indicate  that  the  air  is 
heaviest  in  wet  weather,  if  that  is  not  the  fact  ? 

519.  Is  the  atmosphere  of  the  same  density  on  a  high  hill 
or  mountain  as  in  a  valley  ? 

520.  How  may  the  height  of  mountains  be  ascertained  by 
R  Barometer  ? 

521.  Does  a  person  in  such  elevated  situations  feel  any 
inconvenience  from  the  thinness  of  the  atmosphere  ? 

522.  On  what  principle  is  the  thermometer  constructed  ? 

523.  When  are  two  fluids  of  different  density  in  equili- 
brium ? 

524.  What  causes  water  to  rise  in  a  pump  ? 

525.  How  high  will  it  rise  ? 

526.  What  is  the  construction  of  a  pump  ? 

527.  How  would  you  explain  figure  4,  Plate  XIV.  ? 

528.  On  what  principle  is  a  liquid  sucked  through  a  straw 
or  any  small  tube  ? 

529.  How  would  you  explain  figure  5,  Plate  XIV.  ? 


21 


QUESTIONS  TO  CONVERSATION  XIII. 


ON  WIND  AND  SOUND. 


Of  Wind  in  General ;  Of  the  Trade  Wind;  Of  the 
Periodical  Trade  Winds  ;  Of  the  Aerial  Tides ;  Of 
Sounds  in  General ;  Of  Sonorous  Bodies  ;  Of  Musical 
Sounds  ;   Of  Concord  or  Harmony ^  and  Melody, 


530.  What  is  wind  ? 

531.  How  is  the  air  put  in  motion  so  as  to  produce  wind  ? 

532.  What  is  the  consequence  where  the  winds  from 
different  quarters  meet  or  interfere  ? 

533.  Where  does  this  mostly  happen  ? 

534.  What  regular  wind  prevails  at  the  equator  ? 

535.  Why  is  there  a  regular  east  wind  at  and  near  the 
equator  ? 

^S6,  How  are  the  trade  winds^  as  they  are  called^  pro- 
duced ? 

bSJ,  Why  do  not  the  polar  regions  become  exhausted  of 
air,  if  it  is  continually  blowing  from  them  to  the  equator  ? 

538.  What  familiar  illustration  or  example  can  you  give  of 
the  circulation  of  air — first  from  the  poles  to  the  equator^  and 
then  rising  and  returning  to  the  poles  ? 

539.  Why  are  the  periodical  winds  more  regular  at  sea 
than  on  land  ? 

540.  What  winds  are  called  monsoons  ? 

541.  Why  is  it  that  the  wind  north  of  the  equator  does  not 
regularly  blow  south  ;  and  that  on  the  south  of  the  equator 
regularly  north,  according  to  the  above  hypothesis,  instead  of 
being  almost  continually  variable  as  they  now  are  ? 

542.  What  are  the  sea  breezes,  as  they  are  termed  ? 

543.  Why  does  the  wind  generally  subside  at  the  going 
down  of  the  sun  ? 

544.  Does  the  moon  have  any  effect  on  the  wind  ? 

545.  Is   there  any  difference   of  weight  to  a  column  of 
atmosphere,  when  under  the  influence  of  the  moon's  attrac 
tion.  from  other  times  ? 


ON  \V1ND    AND  SOUND.  243 

546.  Why  is  there  not  ? 

547.  AVhat  produces  sound  ? 

548.  How  can  it  be  shewn  that  air  is  necessary  to  the  pro- 
duction of  sound  ? 

549.  Is  the  atmosphere  the  only  conductor  of  sound  ? 

550.  How  can  it  be  shewn  that   solids  are   conductors   of 
sound  ? 

551.  What  bociies  are  called  sonorous  ? 

552.  To  what  do  they  owe  their  sonorous  property  ? 

553.  How  would  you  explain  figure  6.  Plate  XIV.  ? 

554.  To  what  is  the  tremulous  motion  given  to  the  air  by  a 
sonorous  body  compared  ? 

555.  W^hy  is  motion  more  easily  communicated  to  air  than 
to  water  ? 

556.  AVhy  do  we  see  the  flash  of  a  cannon  at  a  distance, 
before  we  hear  the  report  ? 

5d7'  What  is  the  velocity  of  sound  ? 

558.  How  is  the  sound  of  an  echo  produced  ? 

559.  How  is  harmony  or  concord  in  sounds  produced  ? 

560.  How  is  an  octave  concord  produced  ? 

561.  How  is  that  species  of  harmony  called  a  fifth  pro- 
duced ? 


QUESTIONS  TO  CONVERSATION  XIV. 


OPTICS. 

Of  LinninouSy  Transparent^  and  Opaque  Bodies ;  Of  the 
Radiation  of  Light  ;  Of  Shadoivs  ;  Of  the  Reflection 
of  Light  ;  Opaque  Bodies  seen  only  hy  Reflected  Light  ; 
Vision  Explained  ;  Camera  Ohscura  ;  Image  of  Objects 
on  the  Retina, 


562.  What  is  the  science  called  that  treats  of  vision 

563.  What  is  a  luminous  body  ? 

564.  What  is  an  opaque  body  ? 
^6^\  What  are  transparent  bodies  ? 


244  OPTICS. 

566*  What  are  transparent  bodies  frequently  called,  wheu 
spoken  of  philosophically  ? 

567'  In  what  manner  is  light  projected  from  luminous 
bodies  ? 

568.  Do  the  rays  of  light  which  cross  each  other  impede  * 
each  other's  course  ? 

569.  Why  do  they  not  ? 

570.  What  is  a  ray  of  light  ? 

571.  What  is  a  pencil  of  light  ? 

572.  Is  light  a  substance  composed  of  particles  like  other 
bodies  ? 

573.  In  what  respect  is  liglit  subject  to  the  laws  of  matter  ? 

574.  In  what  respect  is  it  not  subject  to  the  laws  of  matter  ? 

575.  What  is  the  consequence  when  rays  of  light  fall  upon 
an  opaque  body  ? 

576.  What  produces  darkness  ? 
577'  What  produces  a  shadow  ? 

578.  Why  are  shadows  of  different  degrees  of  darkness  ? 

^79'  When  a  shadow^  is  produced  by  the  interruption  of 
rays  of  light  from  a  single  luminous  body,  to  what  is  the 
darkness  of  the  shadow  proportioned  ? 

580.  Why  does  a  total  eclipse  of  the  sun  occasion  a  mor€ 
sensible  darkness  than  midnight  ? 

581.  What  will  be  the  form  of  a  shadow  when  the  lumin- 
ous body  is  larger  thra  the  opaque  body  upon  which  it  shines  ? 

582.  >Vhat  will  be  the  form  of  the  shadow  when  the 
opaqup  Dod}^  is  the  largest  ? 

583.  Why  is  it,  that  shadows  produced  by  the  intervention 
of  terrestrial  objects  in  the  sun  and  moon's  light,  are  of  increas- 
ed size,  instead  of  terminating  in  a  point  according  to  the 
general  principle  ? 

584.  How  may  more  shadows  than  one  be  produced  by  a 
single  opaque  body  ? 

5Sd.  What  is  the  reflection  of  light  ? 

586.  Is  all  the  light  that  falls  upon  an  opaque  body 
reflected  ? 

587.  How  will  a  ray  of  light  be  reflected  that  falls  upon  an 
opaque  body  perpendicularly  ? 

588.  How  will  one  be  reflected  that  falls  upon  an  opaque 
body  obliquely  ? 

589.  What  is  the  angle  of  incidence  ? 

590.  What  is  an  angle  of  reflection  ? 

501.  By  what  rays  do  we  see  opaque  bodies  ? 


OPTICS.  245 

592.  How  are  we  able  to  see  light  that  falls  upon  an 
opaque  body  and  is  reflected,  but  not  in  a  direction  to  meet 
the  eye  ? 

593.  Why  does  one  side  of  an  opaque  body  appear  to  be 
in  the  sunshine  and  the  other  side  in  the  shade ^  when  by  not 
seeing  the  rays  that  fall  upon  the  object,  both  sides  of  it 
would  appear  shaded  ? 

594.  Why  is  it  that  the  whole  of  a  surface  of  water  on 
which  the  sun  shines  does  not  appear  illumined  ? 

595.  Why  is  it  that  objects  on  a  hill  appear  more  distinct 
than  at  an  equal  distance  from  us  in  a  valley  ? 

596.  How  is  it  that  the  rays  of  light  shew  us  or  give  us  an 
idea  of  the  objects  from  whiclithey  proceed  ? 

597.  What  is  camera  obscura  ? 

598.  How  does  a  camera  obscura  represent  the  manner  in 
which  objects  are  pictured  upon  the  retina  of  the  eye  ? 

599.  Why  is  it  that  the  objects  exhibited  by  a  camera 
obscura  are  inverted  ? 

600.  When  an  object  is  pictured  upon  the  retina  of  the 
eye,  how  is  the  idea  of  the  object  conveyed  to  the  mind  ? 

601.  If  objects  are  seen  only  by  their  pictures  on  the  retina 
of  the  eye,  why  do  they  not  appear  inverted,  as  in  the  camera 
obscura  ? 

602.  What  is  the  reason  that  objects  appear  smaller  at  a 
distance  than  they  really  are  ? 

603.  What  is  the  a»gle  of  vision  ? 

604.  What  is  the  size  of  the  angle  of  vision  proportioned 
to? 

605.  Why  is  it  that  objects  of  the  same  size  with  which 
we  are  acquainted  are  known  to  be  of  the  same  size,  if  they 
form  pictures  of  unequal  sizes  upon  the  retina  of  the  eye  ? 

606.  Why  is  it,  that  objects  viewed  in  front  appear  larger 
than  when  viewed  obliquely  ? 

607.  On  what  principle  are  the  laws  of  perspective 
founded  ? 

608.  How  is  nature  to  be  exhibited  in  sculpture  ? 

609.  How  in  painting  ? 

610.  When  are  objects  invisible  ? 

611.  Why  is  it  that  the  motion  of  the  celestial  bodies  is 
invisible  ? 

612.  How  would  objects  appear,  as  to  distance,  to  a  per- 
son,  who  had  always  been  blind,  on  first  being  made  to  see  ? 

613.  Why  would  they  seem  t©  touch  the  eye  ? 

21* 


,.'40  or  TIC.-. 

614.  If  the  image  of  an  object  is  formed  on  the  retina  oi 
each  eye,  why  does  not  the  object  appear  double  ? 

61 5.  When  we  see  the  image  of  an  object  in  a  looking 
glasSj  why  does  it  not  appear  inverted^  as  in  the  camera 
obscura  ? 

616.  Explain  figure  3,  Plate  XVII. 

617.  Why  is  the  ray  C  D  reflected  just  so  as  to  meet  the 
eye  at  A.  ? 

618.  Why  may  we  not  see  ourselves  entire  in  a  looking 
glass,  less  than  half  our  own  length  ? 

61 9-  W^hy  cannot  persons  see  their  own  image  in  a  looking 
glass,  when  they  stand  obliquely  to  the  right  or  left  of  the 
glass  ? 

620.  If  you  stand  obliquely  to  the  right  of  the  glass,  why 
is  it  that  another  person  must  stand  just  as  much  to  the  left  of 
the  glass  in  order  to  see  your  image  ? 

621.  W  hen  you  stand  at  the  right  of  the  glass  and  I  stand 
at  the  left  of  it,  why  does  your  image  appear  directly  oppo- 
site yourself  ? 

G22.  If  all  opaque  bodies  reflect  light,  why  is  it,  that  we 
cannot  as  well  see  ourselves  when  looking  at  any  other  ob- 
ject, as  when  viewing  a  looking  glass  ? 

623.  How  many  kinds  of  mirrors  are  there  used  in  optics  ? 

624.  What  are  they  ? 

625.  How  does  a  convex  mirror  exhibit  an  object  ? 

626.  How  does  a  concave  mirror  ? 

627.  Vv  hat  is  a  focus  in  optics  ? 

628.  What  is  an  imaginary  focus  ? 

629.  Explain  figure  1,  Plate  XVIII. 

630.  Explain  figure  2,  Plate  XVIII. 

631.  What  is  the  focus  of  a  concave  mirror  ? 

632.  Will  the  focus  be  in  the  same  place  whether  the  rays 
fall  parallel  or  converging  upon  the  mirror  ? 

GSS,  Which  focus  is  most  distant  from  the  mirror  ? 
634.  Will  the  focus  be  in  the  same  place  whether  the  rays 
fall  parallel  or  diverging  upon  the  mirror  ? 

Go  J.  Which  will  be  the  farthest  from  the  mirror  ? 
62>6,  W^hat  are  concave  mirrors  sometimes  called  ? 

637.  W^hy  are  they  called  burning  glasses  ? 

638.  Why  does  a  convex  mirror  make  objects  appeal 
smaller  than  they  are  ? 

639.  If  a  light  is  placed  in  the  focus  of  a  concave  mirror, 
how  will  the  rays  fall  upon  the  mirror  ? 


ON  REFHACTICN  AND  COLGflS.  247 

640.  Where  must  the  object  be  placed  in  regard  to  a  con- 
cave mirror  in  order  that  it  may  be  magnified  ? 

641.  Why  is  the  image  in  a  concave  mirror  larger  than 
the  object  ? 


QUESTIONS  TO  CONVERSATION  XVL 


ON  REFRACTION  AND  COLORS. 

Transmission  of  Light  hy  Transparent  Bodies ;  Refrac- 
tion ;  Refraction  of  the  Atmosphere ;  Refraction  of  a 
Lens  ;  Refraction  of  the  Prism  ;  Of  the  Colors  of  Raji^ 
of  Light  ;  Of  the  Colors  of  Bodies. 


642.  What  is  the  refraction  of  light  ? 

643.  When  does  refraction  in  light  take  place  ? 

644.  What  causes  refraction  ? 

645.  Explain  the  figure  1^  Plate  XIX. 

646.  Why  does  the  ray  C.  B.  descend  to  F.  instead  of  D. 
or  E.  ? 

647.  Why  does  a  straight  stick  appear  crooked  when  one 
end  of  it  is  placed  obliquely  in  the  water  ? 

648.  Explain  figure  2,  Plate  XIX. 

649.  Why  is  it,  that  water  in  a  river  or  brook  or  in  any 
vessel  appears  more  shallow  than  it  really  is  ? 

650.  In  what  situation  may  we  view  the  bottom  of  a  water, 
so  that  it  will  appear  of  its  real  depth  ? 

651.  Do  the  sun  and  the  other  heavenly  bodies  appear  to 
us  in  their  real  situation  ? 

652.  Explain  figure  4,  Plate  XIX. 

6ij3,  Why  do  not  the  sun  and  the  other  heavenly  bodies 
appear  in  their  real  situation  ? 

654.  In  what  situation  may  the  sun  be  seen  in  its  true 
place  ? 

655.  Wliat  besides  the  refraction  of  light  is  there  to  pre- 
vent our  seeing  the  sun  in  its  real  situation  ? 

656.  How  long  is  light  in  comhig  from  the  sun  to  the  earth  ? 


^7  18  ON  REFRACTION  AND  COLORS, 

Gd7*  How  is  it  that  our  days  appear  longer  than  they 
really  are  ? 

658.  Is  light  refracted  in  passing  through  a  common  win- 
dow glass  ? 

659.  How  would  you  explain  figure  5,  Plate  XIX.  ? 

660.  Do  objects  seen  through  common  window  glass  ap- 
pear in  a  different  place  from  that  in  which  they  really  are  ? 

661.  Why  do  they  not,  since  they  are  seen  by  rays  of  light 
that  are  refracted  ? 

662.  What  is  a  lens  ? 

663.  In  parallel  rays  that  fall  upon  a  convex  lens,  what 
ones  will  be  refracted  ? 

664.  To  what  will  they  be  refracted  ? 

665.  What  is  the  axis  of  a  lens  ? 

666.  How  would  you  explain  figure  6,  Plate  XIX.  9 
667'  What  is  the  distance  of  the  focus  from  the  surface  of 

the  lens  ? 

668.  What  is  the  object  or  property  of  a  convex  lens  ? 

669.  W^hat  is  the  object  or  property  of  a  concave  lens  ? 

670.  What  is  the  object  of  figure  7,  Plate  XIX.  ? 

671.  What  is  a  radius  ? 

672.  W^hat  is  a  plano-convex  lens  ? 

673.  AVhat  is  a  plano-concave  lens.^ 

674.  How  would  you  explain  figure  2,  Plate  XX.  ? 
675'  What  is  a  prism  ? 

676.  How  would  you  explain  figure  3,  Plate  XX.  ? 
677'  Of  what  do  the  rays  of  the  sun  consist  ? 

678.  How  many  colors  are  there  in  a  ray  of  light  ? 

679.  How  can  these  colors  be  separated  ? 

680.  What  are  the  colors  of  a  ray  of  light  ? 

681.  How  would  you  explain  figure  4,  Plate  XX.  ? 

682.  Why  do  the  colors  of  a  ray  of  light  separate  in  pas- 
sing throur^h  a  prism  ? 

683.  Hew  can  these  colors  once  separated  be  again  united  ? 

684.  What  is  the  object  of  figure  5,  Plate  XX.  ? 

685.  What  composes  white  ? 

686.  What  simple  ilhistration  can  be  given  of  these  seven 
colors  makirg  white  on  beirg  united  ? 

687.  What  causes  the  rainbow  ? 

688.  When  is  a  lens  called  a  burning  glass  ? 

689.  Why  will  a  ^'iece  of  brown  paper  placed  beneath  a 
lens  which  collects  the  sun^s  rays  take  fire  sooner  than  a  piece 
of  white  paper  ? 


OPTICS.  249 

690.  What  colors  do  different  objects  reflect  and  what 
ones  do  they  absorb  ? 

691.  How  do  we  know,  that  opaque  bodies  absorb  all  the 
colors,  that  are  not  their  own,  and  that  they  reflect  their  own 
and  none  others  ? 

692.  Are  colors  essential  properties  of  bodies  ? 

693.  On  what  do  they  depend  ? 

694.  To  what  is  darkness  of  color  owing  ? 

695.  Why  does  blue  often  appear  of  a  greenish  cast  by 
candle  hght  ? 

696.  Why  are  people  of  a  sallow  or  yellow  complexion 
fairer  in  the  night,  if  the  candle  light  gives  all  bodies  a  yel- 
lowish tinge  ? 

697'  W^hy  does  the  sun  appear  red  through  the  clouds  ? 

698.  Why  does  It  appear  red  in  the  morning  ? 

699.  Why  does  the  sky  appear  blue  ? 

700.  Why  is  a  black  di'ess  warmer  than  a  white  one  ? 

701.  Why  is  a  white  one  more  dazzling  than  a  black  one  ? 


QUESTIONS  TO  CONVERSATION  XVIL 


ON  THE  STRUCTURF^  OF  THE  EYE  AND  OPTICAL 
INSTRUMENTS. 

Description  of  the  Eye  ;  Of  the  Image  on  the  Retina ; 
Refraction  of  the  Humors  of  the  Eye  ;  Of  the  Use  of 
Spectacles  ;  Of  the  Single  Microscope  ;  Of  the  Double 
Microscope  ;  Of  the  Solar  Microscope  ;  Magic  Lan- 
thorn;  Refracting  Telescope ;  Refecting  Telescope, 


702.  What  is  the  form  of  the  body  of  the  eye  ? 

703.  W^hat  is  the  sclerotica  of  the  eye  ? 

704.  What  is  the  cornea  of  the  eye  ? 

705.  What  is  the  choroid  of  the  eye  ? 

706.  What  is  the  pupil  of  the  eye  ? 

707.  What  is  the  iris  ? 

708.  Is  the  pupil  always  of  the  same  size  ? 


,30  OPTICS. 

709.  When  does  it  enlarge  ? 

710.  When  is  it  contracted  ? 

711'  Why  does  it  give  the  eyes  pain  on  first  going  into  a 
bright  light  from  a  dark  room  ? 

719.,  Why  does  it  seem  much  darker  on  first  going  out  in 
the  night  than  after  we  have  been  out  a  short  time  ? 

713.  How  much  more  Hght  is  admitted  when  the  pupil  is 
extended  to  the  utmost,  than  when  most  contracted  ? 

714.  Why  can  cats,  horses,  and  some  other  animals,  see  so 
much  better  in  the  night  than  we  can  ? 

71  ^^  How  much  is  it  thought  that  the  pupil  of  their  eyes 
extend  and  contract  ? 

716.  What  becomes  of  the  rays  that  fall  on  the  choroid? 

717.  W^hat  are  the  transparent  substances  called  enclosed 
within  the  membraneous  coverings  ? 

718.  How  many  humors  are  there  ? 

719.  What  are  they  called  ? 

720.  From  what  does  the  aqueous  humor  derive  its  name  ? 

721.  From  what  does  the  chrystalline  humor  derive  its 
name  ? 

722.  From  what  does  the  vitreous  humor  derive  its 
name? 

723.  For  what  are  the  membraneous  coverings  of  the  eye 
chiefly  intended  ? 

724.  Explain  figure  1,  Plate  XXI. 

725.  What  is  the  us«  of  the  refracting  humors  ? 

726.  Explain  figure  3,  Plate  XXI. 

727.  Explain  figure  4,  Plate  XXI. 

728.  How  does  the  refracting  humor  remedy  the  defects 
exhibited  in  figure  4  ? 

729.  Why  is  not  something  like  the  refracting  humors 
necessary  in  the  camera  obscura  ? 

730.  What  causes  some  persons  to  be  short-sighted,  as  it 
is  termed  ? 

731.  Why  can  short-sighted  persons  see  better  by  bringing 
the  objects  near  to  the  eyes  ? 

732.  What  remedy  have  short-sighted  persons  in  viewing 
distant  objects  ? 

733.  Explain  figure  1,  Plate  XXII. 

734.  What  is  the  reason  that  old  people  lose  their  sight  ? 

735.  Explain  figure  2,  Plate  XXII. 

736.  Why  do  old  persons  without  convex  glasses  hold  the 
objects  to  be  seen  at  a  distance  from  the  eye  ? 


QPTICS.  251 

737.  Why  cannot  we  see  distinctly  when  the  object  is 
placed  close  to  the  eye  ? 

738.  Explain  figure  4,  Plate  XXII. 

739.  In  what  way  can   objects  be  seen  distinctly  when 
placed  near  the  eye  ? 

740.  What  is  a  microscope  ? 

741.  Explain  figure  5^  Plate  XXII. 

742.  How  is  the  magnifying  power  in  the   microscope 
increased  ? 

743.  What  is  a  double  microscope? 

744.  How  must  an  object  be  placed  in  regard  to  a  lenS;, 
so  that  the  object  be  magnified  ? 

745.  How  so  that  the  object  be  diminished  .^ 

746.  How  would  you  explain  Figure  1,  Plate  XXIII.  ? 

747.  How  would  you  explain  Figure  2,  Plate  XXIII.  ? 

748.  What  is  the  design  of  Figure  3,  Plate  XXIII.  ? 

749.  How  does  a  magic   lanthorn   differ  from  the  double 
lens  and  mirror  ? 

750.  What  is  a  magic  lanthorn  ? 

751.  What  is  the  design  of  Figure  4,  Plate  XXIII.  ? 

752.  How  would  you  explain  it  ? 


DIRECTIONS 

FOR  PLACING  THE  ENGRAVINGS. 

Plate  I.   to  face  page    31 

II 49 

III 65 

IV 61 

V 68 

VI 78 

VII 89 

VIII 93 

IX 100 

X 110 

XI 114 

XII 118 

XIII 128 

XIV 141 

XV 167 

XVI 173 

XVII 178 

XVIIl 186 

XIX 190 

XX 195 

XXI 205 

XXII 209 

XXIII 212 


